Defining Characteristics
Psionics (mental powers) tends to be less spectacular than magic, fitting into a sort of mythology of the paranormal IRL. Characters tend to have only one or a few psionic powers, as compared to magic where characters may know dozens of spells. It is typically imagined as being rare, and may be unreliable to use.
While often seen as more of an SF thing, AD&D had it (one of lots of extra details to sell it over OD&D), and various fantasy fiction has included psi powers even if not named as such - the idea of "Gifts" in some romantic fantasy series. Some stories or RPGs may also have magic thats' "actually psionics" if trying to give it more scientific plausibility; in RPGs Tunnels and Trolls used this explanation, in fiction there's Stasheff's Warlock series, and perhaps Julian May's Saga of the Exiles books; that last and T&T however have very spectacular "psionics". In fiction, psionics often invokes a lot of superstition and witch-burning, with telepathy in particular having little social acceptability.
 
Power Acquisition
Characters may have a number of racially-predetermined psi powers, a class providing them with powers (possibly increasing with level). Bonus psionic "wild talents" are also sometimes randomly rolled (an old AD&D tradition being to give everyone, or at least those with high mental stats, a base 1% chance of psionics), or may be purchased as an Advantage and/or a skill. Systems may also treat Psionic Ability as an attribute (Space Opera). The "wild talent" rules may affect the design of a psionic class - it may require a high psionics roll or give bonuses to rolls e.g. the original Palladium Mind Mage - or they may not quite fit and have quite different mechanics [the 2E AD&D psionicist]
Examples:
 
*HarnMaster gives characters a number of powers rolled based on their Aura attribute (which governed both magic and psionics); each power known is then treated as a skill with its own %. A power usually starts at a low % (making it dormant); Incidents triggering it accidentally might result in improvement checks, until it reaches a certain threshold, at which point a character becomes aware of it and so can train it up normally via practice. Powers scale up based on the character's Skill Index (1/10th of skill).
 
*Masque of the Red Death for 2nd Edition, IIRC, let characters buy "mentalism" proficiencies with non-weapon proficiency slots. This to an extent gives high INT characters more psionic powers, as they have more skill slots.
 
*Rifts gives characters a roll for minor or major psionics, as well as having "racial character classes" with more powers known; a number of classes had a few minor powers or a greater chance than normal of psionics. Especially strong powers are grouped into the [Super] psionics category.
 
*Savage Worlds lets characters learn about 3 powers by taking a Arcane Background edge. Using their powers requires a separate Skill. Adding extra powers requires an 'extra power' Edge (giving a single extra power - very conservative, and the system ends up being fairly granular i.e. very limited 'cantrips' or non-combat magic).
 
*GURPS/HERO has character buy powers with character points.
 
*Space Opera as noted treats Psionics as an attribute; classes receive some discretionary points to spent on specific attributes, possibly letting some classes build their psi powers; it could also receive racial bonuses.
(there was a "Psiworld" RPG from the same company, FGU, which I'd imagine probably worked similarly, but haven't seen).
 
*AD&D rolled for psionics randomly; however it limited certain "wild talent" powers by main class for balance reasons i.e. no skin armour for wizards.
 
*2E D&D as well as having a random roll for "wild talents" has a Psionicist class with access to psi powers. Powers are divided into "sciences" (major powers) and "devotions" (minor powers); with more powerful sciences tending to have more complex prerequisite powers. This has the nice effect that a character who is high-level can continue picking up weaker powers, as well as more stronger powers.
Comparatively 3E just gives a "total number of powers known" so once a new power level was achieved, your picks would (optimally) all be of the highest-level powers. 3E D&D also revised powers to be balanced much like spells i.e. there were 1st-level psionic powers through to 9th-level psionic powers, largely slight reskinnings of magic spells and making psions not very different to sorcerers.
 
*Monsters in 3E are interesting in that they treated innate monster ability psi-powers differently to class-based psionics; monsters had "psi-like abilities" (useable x/day or at-will) instead of power points. These are sometimes available to PCs e.g. as racial abilities of psionic races; most of the psionic races get bonus Power Points, but can't do anything with these power points unless they choose a psionic class as well.
 
 
Usage Limitations
Nearly all psionics systems use some sort of system of "power points" to use their powers - even D&D, despite Gary's dislike of "spell points" (as noted in his book Role Playing Mastery). Exceptions/complications:
 
*HarnMaster instead of points has characters roll a check to avoid gaining physical fatigue (measured in "fatigue levels"). Each level gives -5% to all skills.
 
*Talislanta 4E gives characters a skill check to use magic or psionics ("Mysticism"), with a cumulative penalty equal to number of spells cast. This makes power use increasingly likely to result in critical fumbles.
 
*2E AD&D has a character make power checks to see if a power works, with a failed power roll meaning that 1/2 the points are expended. A particular problem power where they didn't think this through is "Dream Travel" letting a party travel via dreams; if a power check is failed, the journey works but is 10% short per point the roll missed by, so failing the roll by exactly 1 by RAW gets a party 90% of the way for half cost. (i.e. this power needs a special proviso that its always full-cost).

*In Dragon Warriors, Mystics make a check to avoid "Psychic Fatigue" - this is a d20 roll under [13 +mystic level - power level]. In comparison, DW Sorcerors have simple level-based "spell points" ; the fatigue roll gives a mystic on average a similar # of highest-level spells to sorcerors, but casting a lower-level spell is still very likely to burn out a mystic, whereas the sorceror can safely output lots of low-level spells.
[In a homebrew version of this I was working on, I was planning on bumping Mystics up to 3 failed fatigue checks/day at target 11+mystic level- power level, using a 2d10 roll instead of d20, and giving sorcerors a linear spell point progression of 7 points/level; about 3 spell levels doubled chance of fatigue, that led to a power level equivalence as follows:
 

Level Mystic Sorcerer
1        0.75   0.5
2          1      1
3                1.5
4                 2
5          2     2.5
6                 3
7               3.5
8          4      4
9         ---   4.5
10        ---    5

 
Note as with hit points, power points may be arbitrary, or may have some other use in the system.
i.e.
 
*IIRC, LegendQuest has separate "mental fatigue" and "physical damage" totals, with Magic (including Mysticism) dealing mental fatigue damage. A proportion of either lost can result in an action penalty.
 
* a system with a Willpower stat could possibly have psi use have a Willpower cost. [World of Synnibarr gives psis power points, but also lets characters burn Constitution points to amplify powers, regardless of whether they're magical, mutation-based, chi-based or psionic; I'd be tempted to modify this to have psionic classes burn Wisdom to fuel their powers in place of Con, to give more differentiation between the power types, if I ever played it].
 
Power point recovery may require full bed rest/meditation, or there may be fixed regeneration of points/hour e.g. 2nd ed. has a table of recovery rate by exertion level. This limited impact of point depletion on need to rest, but could add to accounting. (Tunnels and Trolls magic system is similar; point expenditure here actually reduced character Strength and so combat ability, but conversely GM encounter guidelines encourage wandering monster rolls while PCs attempt to rest. Warrior types could potentially lose STR in combat as well, albeit not as quickly, from using overly heavy weapons or berserking).
 
 
Controlling Attributes for Psionics
Some games have a "psionics" attribute: Aura in HarnMaster, Psionics in Space Opera; Psychic Talent in Dragon Warriors. Wisdom/willpower or an equivalent is the usual prime requisite in other systems, e.g. in Palladium, Mental Endurance is a main factor in determining "Inner Strength Points". Occasionally Intelligence may be used, instead.
Some games also have more complex setups e.g. AD&D modified 'psionic strength' based off all of Int/Wis/Cha.
3E assigned one "discipline" of psionic powers to each attribute, giving some psions Str/Dex/Con based powers - Str for psychokinesis, Dex for psychoportation or Con for psychometabolism, Int for shaping, Wis for clairsentience, Cha for telepathy. 3.5 removed the stat/discipline correlation which psionics based off just one of Int, Wis or Cha, depending on class; it also uses wisdom as a prerequisite for some feats e.g. Rapid Meditation. SW psionics is independent of any attribute, despite there being a "Spirit" stat.
 
2E powers used an ability check to see if powers worked (typically of Int, Wis, or Con) - a new mechanic compared to 1st edition inherited from the skill (Non Weapon Proficiency) rules. For why this is a bad idea, see mutant powers, above; additionally, unlike most classes, Psionicists do not improve at any of their powers as they level up. They gain more powers known and power points but a 3rd level psionicist with Con 18 and disintegrate would have a higher success chance for using it than a 20th level psionicist with a 13 Con and the same power. Psionic Monsters in 2E have to have default listed power scores for psi powers rather than using an ability check, as they have no defined ability scores.
 
Mental Combat
Games may also include “mental combat” rules. 2nd edition D&D had various attack and defense modes, which had different modifiers against each other somewhat similar to the earlier 1e weapons-vs.-armour-type tables. Some games may also have “mental hit points” that may be damaged by psychic attacks. (note that this has tactical effects: psionic and physical attacks vs. a single opponent no longer have any synergy, though parties still function when facing lots of small opponents). Fairly often a mental attack will just do HP damage for simplicity's sake - e.g. 2E "psychic crush" deals damage no different to a sword's, while 4E and 5E do damage normally off HPs although with a "psychic" keyword that comes into play for defining resistances etc. Games where physical damage is subtracted directly from an attribute e.g. CON can in a fairly balanced way subtract mental damage from a mental attribute like INT or WIS - a particularly elaborate system for this is DC Heroes. A "partial synergy" can be established if the 'HP' pools are separated but with damage to either mitigateable by spending a central pool of metagame points - again DC Heroes is an example, where Hero Points can be spent to reduce damage against whichever attribute.

Chi rules (or Ki, the Japanese version) are included in only a few games with an Eastern or martial arts flavour. (At least few that I know of).

*World of Synnibarr:  the author's desire to make everything super-awesome, and interest in martial arts, leads to inclusion of a Ninja class and a "chi" power type. Chi can however do basically anything here: e.g. Ninjas can teleport (Tesseract) with it, or produce light sabres, while the infamous flying grizzly has a laser eye beam that is a chi-based energy attack. Chi's basic advantage in the system is that as it uses internal energy, it is difficult to suppress, unlike magic-based or mutation-based powers which are both envisaged as using an external energy source, "caprenium radiation".
Chi can still be suppressed in unusual circumstances e.g. certain coloured suns can suppress it (different star colours affect other power types).

*Ninjas & Superspies gives characters a Chi score (base value = Physical Endurance; some forms may double this, initially or after some levels) which powers various martial arts abilities; compare to Psionics which is powered by Mental Endurance or Magic which uses a separately rolled PPE (Potential Psychic Energy) score.
Positive Chi is required for normal healing; there are "negative Chi" abilities which require a character replace their normal chi with dark chi temporarily. The dreaded "Dim mak" ability prevents characters from recovering chi normally, leading to eventual death. The softer or more spiritual martial arts forms have more Chi powers; a number of chi attacks are delivered with 1-finger strikes.

*D&D 3.x has a monk class which has ki powers; most of these operate using the standard spell-like ability rules.

*The Way of the Tiger gamebooks had "Inner Force" points which could be spent to double damage from martial arts attacks.

*Weapons of the Gods: I've only glanced at this and mostly forgotten any details but WotG's main contribution to Chi, that I recall, is that gay characters are forced to take a particular discipline to prevent themselves getting Chi depletion.
(WotG is also notable for having the fairly bad idea of levels (ranks) that go backward from 5th (most crap) to 1st (awesome); leading to a BBEG who is off the scale with a level of "below 1st rank". (Gygax's Lejendary Adventures did the same thing, with 13th rank being the worst.).
Weapons of the Gods, the first edition only apparently, has about five different pools for 'Chi', all of which regenerate at the same rate but promoting switching-up strategies to not bleed any particular pool too dry.

Next time: onward to Magic, I think
..

“Sadira kept her hand open. One after the other, the cacti drooped, then browned and withered. . . Even then, she did not stop, until the soil itself turned black and lifeless.
...Though the sorceress believed she had been justified in saving herself then, the present issue was less clear. . . If she resorted to defiler magic to save herself from eventual death, would she use it out of simple convenience the next time?”  
-       The Amber Enchantress (Troy Denning).
   
Magic is perhaps the game subsystem with the widest latitude for the prospective game designer. Most other game systems must correspond in some fashion to reality; a magic system is however entirely fanciful and its basic principles of operation can be anything the designer wishes. Fiction contains any number of basic ideas for how it works, its limitations and so on, which can translate to any number of game mechanics.

Magic might be the direct rewriting of reality by belief which requires a spell only because the wizard believes it does (as in Mage: the Ascension), or it might be a call to spirits or other-planar creatures that are compelled to obey the instructions, or a speaking of true names of the things being affected (a la Earthsea), or even a form of psionics but where the wizard needs to speak certain words to "focus" themselves, or something.

Magic itself may be easy or difficult to learn, work perfectly or only sporadically (a 'klutz roll'/skill roll), and be based on various attributes. Most systems (and much source fiction) places limits on how much magic a wizard can use: this can include spell memorization (e.g. Jack Vance or Terry Pratchett), mental or physical exhaustion (perhaps suggesting some form of spell points, or even a tie in to fatigue points or a condition track), or even require some sort of energy from the environment; Dark Sun where magic sucks dry plant life had an interesting concept which was however poorly represented within D&D mechanics, while in fiction the idea of mana being a finite resource being steadily depleted by wizards is explored in some depth in a number of Larry Niven’s works (e.g. ‘What good is a glass dagger?’; or ‘The Magic Goes Away’). The Gaming Den has tossed around the idea of a "winds of fate" matrix for power use, with a character able to use only a subset of powers from their matrix each round. Use of magic can also have moral or at least roleplaying consequences.


Magic use in source material is sometimes linked to characters personality/feelings, though that's sometimes an issue in that game systems leave that largely to GM fiat. (for instance, Star Wars D6 has a 'Concentration' Force power with difficulty to use based on if the character is currently angry or fearful, something fuzzy the game mechanics are otherwise silent on and so possibly requiring GM adjudication of what a PC is currently feeling).
Apart from resource usage, the behind the scenes limitations and principles of how magic works are often fairly badly defined.  (D&D 3E is a particular offender in this regard with magic that can basically do anything; my pet hate being that even though the combat system doesn’t handle specific injuries – meaning you can’t cut off someone’s hand or head with a frickin’ sword, at least unless its a magic sword – there are spells which can do it, like Grim Revenge or Decapitating Scarf).

In RPGs, Ars Magica sets very clear limits to what magic can accomplish – such as not being able to interfere with souls (and, thus, resurrect the dead), interfere with the divine, or affect ‘spheres above the Lunar’ ; each variety of magic (all PCs use just the one, ‘hermetic’ magic) can break one law. In fiction, another interesting set of principles rather than limitations may be the one from Lyndon Hardy’s ‘Master of the Five Magics’ where each style of magic has underlying rules, if not limits(originally derived from computer RPG 'A Bards Tale').

Another essay on guiding principles in magic which may be of interest is this one by John Kim (discussing how to make magic more evocative of myth and folklore): http://www.darkshire.net/jhkim/rpg/magic/antiscience.html

Just as super powers systems tend to use references to super powers as a shortcut in designing, fantasy games tend to use references to magic. Rolemaster had classes partly defined through new 'spell lists'; D&D (most versions) use the spell system as a library of standard conditions, good and bad, which can lead to warriors' speed being described as being like use of a Haste spell or rangers getting 'spells' to represent special abilities that may have been intended to be special but not precisely magical. (forum post by one of the designers of FantasyCraft here on this). (Occasionally something gets twisted fairly oddly by this e.g. Sharn in 3.0 D&D get 'haste' as an ability -an extra 'partial action'- to represent their three heads' ability to let them cast 2 spells/round).

Magical spells are often broken down into 'levels' restricting access to stronger effects.

"If you belong to the class of the Warrior then set aside this text, for the touch of iron is harsh in your blood, and you will never cast a spell." – the Book of Magic, Crusaders of Khazan
 
 
 
Whether a character can use magic may be an innate 'gift' of some kind and/or require a particular class or skills. Some variants include:
 
 
*magic-use limited by class. If multiclassing is allowed, most any character can theoretically use magic with additional training. In the case of D&D through to 3E, different classes require different key attributes; a character's maximum ability score determines what spells they can cast, and so whether characters have any magical potential. Generally though, limiting spells to a particular class is enough to keep it rare. (A couple of weirder class systems deliberately scorn niche protection and let anyone use magic anyway - e.g. Rolemaster would let your fighter learn spell lists, but at greater skill point costs. I'm not sure this doesn't miss the entire point of a class system, though).
 
*a 'Power' attribute. For most systems which have these they largely determine how much magic a character can use, but it can potentially also determine if a character can. Odds of magic use in the general population can then be related to likely power ratings (although PCs may well roll Power in an unusual way). Power as an attribute might do very little for non-mages (in point-buy systems, this gives the mundane an opportunity to get a leg up with regard to other attributes like Str or Con), or it might influence e.g. whether a character can use certain magic items (e.g. in Dragon Warriors, a character with 8 or less Psychic Talent, cannot use magic rings) or a character’s resistance to magic/saving throws.
 
*Talislanta characters have Magic ratings determining raw magical ability - magic itself is a d20 skill roll (certain classes get this as part of their classes, while others may be able to learn it separately). Certain racial types are also totally unable to use magic; the warrior race of thralls are racially incapable of understanding it, while dual-enchephalic Sindarins lose one of their brains (and several Int points) if they use magic.
 
*Tunnels and Trolls magical ability is a surprisingly complex affair; spells require you to be the right Type (Class) to use magic, and have a minimum IQ and DEX depending on the spells' level. While the game has character levels, these don’t affect what level of spell a character can use – only what level of spell NPCs are willing to teach the character. Later editions also have a Power (“Kremm”) attribute which works as spell points; Warriors do still have a Power attribute though it only determines their magic resistance.
 
*Other games have a "Magic rating" only for wizards. Mage: the Ascension gives mages an "Arete" rating representing their understanding of "universal Truth" (in Mage, the idea that reality is a consensual hallucination the mage can control); this IIRC limits their maximum Sphere ratings (in different magic types). 1 point of Arete makes a character a Mage, rather than a normal human ("sleeper"). This sort of mechanic also showcases how a number of Storyteller games have an expensive “Uber Stat” which correlates highly with character power, sort of like level but not quite – Generation, Rank and Quantum being some others of these for different types of character.
 
*Magic use might be a Merit of some sort: this is particularly likely in games which are purely skill-based rather than class-based, where otherwise every character might pick up a few ranks in sorcery.
 
*Magic use might even be a racial feature; not a common approach, but Kevin J. Anderson's fantasy novel "Gamearth" has an RPG world where only characters descended from the 'Sorceror' race can use spells, with the proportion of sorcerer blood determining amount of magical ability.

*In some games a characters magical ability might change over time; for example a couple of science fantasy games make cyberware or bionics off-limits to wizards. Shadowrun gives characters an "Essence" rating, starting at 6; if this falls due to Cyberware characters also lose Magic points (rounding down). This limits the maximum Force of spell a mage can cast without risking physical damage, rather than just mental fatigue (at least in 1E SR).
Rifts likewise has magic depletion for cyberware , but without a defined Essence mechanic; cybernetics reduces magical ability (range/effect) by 25%, while extensive bionics forces a change to the Cyborg ('Borg') OCC, removing all spellcasting and reducing Potential Psychic Energy (PPE) to d4.
Synnibarr lets characters have up to 75% 'biomass replaced' before affecting magic use or other powers, totally removing it. Extra limbs and wings etc. could give characters extra biomass (>100% to start), letting them fit in additional bionics and still cast spells. % biomass lost also cost that proportion of base hit points.
(An opposite approach that could work might be for magic to interfere with cyberware, instead of cyberware with magic. For instance in fiction, Jim Butcher's Harry Dresden books' title character has magic that interferes with electrical equipment nearby - this was then also cause problems with any sort of implant, not that its an issue in his mileau.)
While on the topic of cyberware, Cyberpunk has point depletion for cyberware as well (it costs Humanity points, calculated off [Empathy attribute x 10] –though it has no magic this at least slows down the entire party becoming robots. With the peculiar feature that combat characters start off with high Empathy scores i.e. are Gandhi until they finish cybering up.
Battlelords of the 23rd Century has cyberware costing Constitution points (probably easier than the % multiplication of base hit points for Synnibarr). (I don't recall if there are bonus 'HPs' for cyberware to offset the loss of 'meat' points - possibly).
Heroes Unlimited has just expensive bionics (borgs get a several million dollar budget). A couple of games have 'bioware' systems - I think modern Shadowrun includes a few (lower essence costs), while the Amazing Engine game Kromosome was built around it. Rifts Atlantis has 'bio-wizardry' which is basically identical to Rifts bionics, but the only class with built-in 'bio-borg' abilities is optional (Conservator).

Note that the more character generation resources (class, race, levels, skill points, stat points, merits, etc.) are burned up to become a spellcaster, the more reliant a character is forced to be on magic. If magic is a total investment, it must also be frequently used for overcoming obstacles.  

 

Excellent, I was a bit afraid you dropped this thread . Will you post more on the subject of magic? (if such, tell me, I'll remove this post for cohesion).

Just busy post-holidays, and I was running a game (trying out Savage Worlds) over the weekend and stuff. Definitely more coming on magic - you can delete the post if you like, though then I'll have to edit this post into something useful so it doesn't seem like I'm talking to myself

Magic may sometimes involve a skill roll, or have a set chance of failure (e.g. gamebooks like Demonspawn series giving magic a "fundamental failure rate". 2E D&D had % spell failure for clerics based purely on their Wisdom  score, irrespective of level; perhaps representing an idea that the  (foolish) PC will have succumbed to some temptation requiring their  deity to punish them by holding out on granting a miracle.
Difficult circumstances may also add extra chances of failure e.g. D&D 3.5's "Combat Casting" rolls and % spell failure due to armour. Mage has consequences for casting spells near "sleepers" who don't believe in it - this causes accumulation of Paradox points and potential paradox backlash.

*A spell may also allow a separate saving roll, and/or require an attack vs. a target creatures' defense number (Armour class or what-have-you). For more on saving rolls see Adventuring Situations and Combat Actions - Defensive actions. Usually each time a spell is cast a save is allowed (the point being that they get at least a fighting chance) although Arduin was noteworthy in allowing saves once only - if a save vs. fireball fails, the target will always be susceptible (until their level improves), or if it succeeds will always succeed (unless the wizards' save improves) - this also went against the idea of the save representing 'dodging' instead saying that 'the mage's fireball is not quite the right brand of magic to 'cook' our orc with'. On the plus side this does incentivize changing up tactics and using different spells, rather than repeating a save-or-die until they fail.

*Systems can also require a roll to see how well a spell worked; D&D wild mages for instance roll for caster level variation in their spells, which in 2E required checking a table. 3E replaced this with "lower caster level by 3 and add d6 with each spell", a more elegant mechanic which was however immediately abused by munchkins who didn't particularly care for wild magic as a concept, but figured out that Practiced Spellcaster would cancel the -3 and just give them a caster level bonus.

*Spellcasting can also have a roll to determine not success, but amount of 'fatigue' or lost magic points for casting a spell; miscasting (spell fizzle) might occur only on a critical fumble, rather than a normal failure i.e. comparing this with a normal "Unreliable" magic system:
[U]Result:[/U]              [U]Unreliable Magic[/U]        [U]Variable Fatigue [/U]
Critical Failure      Caster becomes walrus       Spell Fails
Failure                  Spell Fails              Spell Works, extra cost
Success                Spell Works               Spell Works, normal cost
Crit. Success         Spell works really well     Spell works, low cost
Spells may also include some form of fumble table, to handle critical failures on your spellcasting roll (if these are not purely up to the GM to imagine). Depending on system these might be light-hearted or deadly, precisely defined or merely suggestive.

*2E D&D wild magic included a d100 table of 100 random effects, with 01-50 being "affects caster" results and 51-00 being "affects target" results. The table was constructed so that adding a character's level to the roll was usually a bonus (00 or better indicating spell works at triple effect). Protection from wild surge spells also sometimes allowed a character to roll twice and take the best result.

*Talislanta magic uses its generic action table where 1s (on d20) are  mishaps. When I played this, these happened very often, including some  nasty friendly fire casualties - but I'll grant this may just have been  dick GMing. Mishaps left up to GM interpretation left them free to plug in their own mishap table.

*2E's psionics system used a d20 roll under ability (Blackjack method i.e. higher under your score is better) and for each power defined 20 (fumble), Power Score (exceptional success) and often 1 (marginal success) results. Similarly, Masque of the Red Death required a Spellcraft NWP check for spellcasting by 'adepts' (Wizards). Both of these are problematic due to making a class feature level-independent, and strongly encouraging maximizing a score (it tries to fix this by spreading checks around 3 scores - Wis/Int/Con; still, psionicists vary in power enormously depending on whether a game uses a weaker or stronger stat generation method). The check to see if the power worked, was often also the roll the target has to roll against, although only if they were making an ability check (d20 under stat) - saves use a different mechanic and so couldn't get a modifier worked out except rarely (e.g. a power score result with disintegration penalizes the result by -5). Powers failed frequently ('double jeopardy') since a successful roll might still require an attack roll as well. Another quirk is that a failed power roll cost 1/2 PSPs but powers also failed if the opponent's resistance roll passes, but at full cost.

*Different Worlds #2 describes the cult of Cacodemon for Runequest; initiates can summon their master with a roll under POW on d100; if successful it costs the amount of POW rolled, so it "eats the POW of the initiate entirely if the exact amount of POW is rolled" (i.e. it eats their soul or something).

*original Advanced Fighting Fantasy has an "Oops!" only on snake eyes on 2d6, with a roll on the table including the nasty "caster disappears leaving only a pair of smoking boots" result.

*Arduin spell failures may give reverse results for spells - with catastrophic results. The example of a "Magick Phumble" in the book for a lowly Create Water has it destroying an equivalent volume of water instead - giving an almost certain TPK as everyone in the area starting with the caster suffers a Horrid Wilting type death via dehydration.
Arduin tends to design spells to frequently be unreliable with various specific, individual spell-by-spell conditions or caveats, as well as countermeasures.

*Rolemaster, as always, has tables for this, not just for Oops but for varying degrees of success.

*reportedly, Mark Swanson invented a 'klutz' system for 0D&D where each spell had a chance of failure, increasingly cumulatively each time it was used (other spells tracked % separately). If klutzed the roll was checked again to see if the spell 'double-klutzed' i.e. rebound on caster.

*in Fantasy Wargaming, Spellcasting has a 'staged resolution' system where a wizard must establish a 'link' to a target before casting a spell. A magic-using target can resist by sending a 'desist' or 'counterspell' back through the link before the attacker's spell is cast. See here for notes - http://www.skotos.net/articles/BSTG_54.shtml

*the Demonspawn books have a roll for individual spells to work, plus also have a roll at the start of a section where the hero (a barbarian with a distaste for magic) determines if they can stomach using it...as a character-specific flaw, a similar roll could be duplicated in other games with disadvantage rules, e.g. a DC Heroes character could have a 'minor irrational attraction' to not using magic.

*Ravenloft for 2E AD&D requires a d% 'powers check' to see if the Dark Powers notice a spell being cast and interfere.

[EDIT NOTE] * Potentially a spell could have a guaranteed basic effect without a check, but with skill checks be required to add extra add-ons or increase the effect - an interesting compromise in reliability.

Most systems limit how often magic can be used. Some methods...

# Spells Per Day: Palladium FRPG (the original one) and Heroes Unlimited versions of Palladium used this; characters can cast a fixed number of spells, chosen as they go, regardless of which spell this is. Simple book keeping is fairly simple but spells must be designed more carefully to prevent one spell being too good/overused; they might be balanced by varying chances of success, casting time, or exceptional resource costs like a GP cost. Very powerful spells might have a cost of (2) or (3) normal spells, making this into a crude spell point system. Generally however, this system avoids the problem where a character might just spend their entire day's spell points to nuke everything in an imbalanced way, and then be useless/need to rest immediately.

Vancian memorization: this gives a character high-level spells alongside low-level spells, meaning that characters have a high rate of advancement. The spell point characters point reserve might give them a 2nd level spell instead of their existing 1st level spells; the memorization system usually gives a character a 2nd level spell as well as their 1st level spells.
The main drawback of spell memorization for the caster is that spells must be pre-prepared to suit the situation; this lets spells have somewhat greater effects than they otherwise would have, without being unbalanced. A character with their 1st level spell may have to choose between Shield (protection vs. enemy magic users with magic missile), Sleep (zaps enemy fighters), or Tenser's Floating Disc (letting you carry out a hoard of copper pieces). This rewards intelligent play, but adds excessive book keeping (a particular nuisance with NPCs) as more spells pile up.
4E D&D is essentially this for most classes, though some resources instead function per encounter or at-will.
The pre-preparation involved in Vancian memorization also sometimes has meant spells are designed to be individually more versatile i.e. yes you took Otiluke's Freezing Sphere but it can be cast in a couple of different ways to do somewhat different things. OTOH, systems without preparation are more likely to divide the sub-effects into different spells for more clarify in the spell description- compare 3E dispel magic with 5E's separate dispel magic and counterspell, now that the opportunity cost of taking both is relatively reduced.
Have various 'spell slots' of different levels lets mechanics hang off level exchanges in a way that spell points can't. For instance, one obscure 3E sourcebook has a 'Netherese Arcanist' feat that lets a character break down a higher level spell into multiple lower-level slots (3 1st in place of one 3rd-level spell, for instance).
As with spells-per-day, this prevents characters burning all their spell points in one go, though with more complexity than that.

Recharge Magic: an obscure 3.5 optional rule (Unearthed Arcana) - something similar is more common in MMOs (cooldown timers). This makes a spell "offline" for a while after use, how long depending on its level.

Progressive Check Penalties: Talislanta 4E uses a check to cast spells, with a cumulative -1 per prior spell cast. This forces characters to move to lower-level spells as their day progresses or suffer an increasing chance of a mishap occurring.

The Blood Sword method: this series of gamebooks had an adaptation of spell memorization where a wizard had to ready a spell into memory before it could be used (taking an action), and had to make a spellcasting roll to get off a spell; a character could adventure with some spells pre-readied to avoid this extra time, but each spell readied gave them a -1 to Psychic Ability, i.e. a penalty on their rolls to cast spells. Prepared spells could be changed freely in between encounters. Player choices were further complicated by spells having different casting penalties, and the possibility of the spellcasting stat being used for other game functions (i.e. rolled against to resist enemy psychic attacks).

Spell Points: while only slightly harder to keep track of than spells per day, these allow for spells that use more or less power.
Spell points are likely to be burned through quickly. Unless the character knows they need to conserve power (and is unable to rest) then the most powerful spells tend to be used first. Making this more tactically interesting can be done by either decreasing return on cost for more powerful spells (a 2nd level spell costs many more points than a 1st level spell, but isn't much better) or adding penalties for using up magic - for example, in Tunnels and Trolls through to 5th edition, casting spells cost Strength points which would immediately lower a characters combat ability including what weapons they could use.
As all a wizard's spells are available at any time, there may be some chance of decision paralysis depending on the player and how many spells are involved. Spamming the same damaging spell over and over is also possible, unless targets can take precautions against them or the spells themselves vary in effectiveness based on terrain and enemy type.

Systems may have fixed spell point costs for a given spell level (i.e. all 3rd level spells cost 5 points); in others each spell has its own unique cost (slightly more book keeping; more likely if characters have a lot of spell points since otherwise there isn't room for much variation). Costs may be fixed, modified by spellcasting roll (see prior post), have slight random variation, or have a fixed subtraction for higher-level characters, giving high-level wizards who can cast cantrip-type spells more or less at will. This principle can also be used if a high level character should be able to use more spells, but where spells drain a resource that doesn't increase with level, like an ability score (as with T&T characters).
Some other spell point related odds and ends:

*Dragon Warriors lets Sorcerers cancel durational spells to regain 1/2 the used magic points. Spells in DW have a variable duration (roll an expiry check as either 12 on 2d6 each round or 25% per minute), so the character has an incentive to cancel used spells as soon as possible. One rulebook adventure for DW has a trapped corridor which drained Magic Points from victims; clever players could avoid this by layering on as many durational spells as possible, walking through, then cancelling them.

*Rifts has a pretty evocative magic system. Spell points ('Potential Psychic Energy') are possessed by nonwizards, but can be donated willingly or forcibly taken (they are doubled at the moment of death) by sacrifice. Ley lines (particularly at conjunctions) can also provide PPE.

*Assigning costs for spells is a fairly ad hoc process, but SenZar has a fixed "1 power point = 1 hit point" rule for damaging spells, with spell level increasing the damage cap. (This would also work well for determining spell points a character gains if they have a special power to defensively 'absorb' magic.)
A system with GURPS-esque merits and flaws could also relate the cost of spells that give a target an advantage or disadvantage to the advantage cost (i.e. if Blindness is a 5-pt flaw, blinding an enemy will cost 5 magic points).
Another interesting 'effect-based' costing was Tunnels & Trolls Omniflex, letting characters rearrange their permanent attribute scores (keeping the same total). 5E gives this a flat cost of 186 temporary Strength (impossible to meet without significant magical stat gains, special cost reductions, or a cooperative ritual); Deluxe instead gives it a Wizardry cost equal to the # points moved from one stat to another.

'Powering up' spells is sometimes allowed for extra spell points. Arduin Adventure lets characters do this for example - paying the full cost of the spell for +50% effect - spells are cheaper enough that this tends to be fairly broken (potential for any wizard who could cast fireball to do it at x4 power for triple the Hit Points of any PC). While the cost was higher, the spell's 'level' didn't change. Tunnels and Trolls, or 5th Edition D&D, let characters 'level up' spells which costs more points / a bigger slot - in these cases with max. effect automatically limited by what spell levels a character can use.

*Monsters or magic items may also siphon off spell points e.g. cerebral parasites for psionicists in D&D.

*The Fantasy Trip and psionics in 2E AD&D charge extra spell points for durational spells while these are running ('maintenance costs).

*5E D&D uses both spell level "slots" for sorcerers and points ('sorcery points') which can be used to pump up spells, converted back into extra spell slots or vice versa (at a poorer conversion rate). Since conversion runs both ways, a sorcerer could use it to change lower-level slots to higher-level slots but inefficiently (e.g. 3 1st = 3 sorcery points = 1 2nd). This also lets sorcerers have more spells than wizards, despite primary casters sharing the same spells/day table to make multiclassing easier.
5E also has an interesting 'encounter power' mechanic in that characters can sometimes get class abilities recharged when rolling initiative if at 0 points (Relentless & manuever dice, Perfect Body & Ki), guaranteeing some use in any combat.

Exceptional Resource Drain: In all of the various systems, exceptional resources may act as an added balancing factor for specific spells. These can include material components with a non-negligible cost, experience points in some systems, or drains to a character (i.e. loss of Constitution points or hit points which might be permanently lost, temporarily lost, borrowed or shared with creatures or constructs); sometimes these costs may be borne by a beneficiary rather than the caster.

Resource usage mechanics are tricky in that they can encourage or enforce a particular encounter workday that character's shouldn't go beyond. Avoiding one set of adventure design problems by having characters always be at maximum fighting strength after an encounter (and hence not wanting to rest) unfortunately generates another set of problems, in that weaker encounters wearing down resources are no longer relevant, so that encounters need to be at the characters' power level exactly (4E D&D).

As with other skills, magical skills can be defined very narrowly or quite broadly. E.g.
 
* A single magic skill. A class based system is more or less this – usually all varieties of magic cast by the character will use the same level, though odd classes may have multiple separate spell lists/caster levels.
 
*skill levels applying to a single branch of magic (a “school” or “mode” etc) i.e. all conjuration type spells might belong to one school, blasts to another and enchantments to a third. Systems letting characters construct spells on the fly might define spell “seeds”, customizable on the fly into specific spells; which may have individual ratings.
 
* In Ars Magicas “nounverber” system, a spell requires two aspects (Form & Technique) with separate bonuses; a spell adds the two most applicable scores i.e. a blast of fire would use Creo (create) + Ignem (fire).
 
*skill levels can be in individual spells. LegendQuest lets characters buy “control levels” in specific spells, which are divided as the character wishes between effect, % success, and range. (It does also have a general magic skill, but this helps only with powering spells). (IMHO, while LQs splitting of control levels is an interesting mechanic, this setup limits wizards to a few spells over and over and so may be s bit dull; the same idea can be applied to a system with just one magic skill – it would also work very well with a dice pool system like that of Heroic Golden Turbulence).
 
*systems with skill specialization rules can have characters who have general magic ratings, but higher values in specific schools/spells, even where “Magic” is a single skill.
Aside from skill specialization, a game system might also have multiple wizard specialties off a single Magic skill by giving spells benefits from different character attributes i.e. a system might have a raw Power attribute that modifies spell damage and an Intelligence modifier for illusions and subtler enchantments (perhaps effects involving saving throws), leading wizards with the same spell magic skill and spell lists to still prefer different spells. Merits/flaws can also modify a wizard’s preferences.
 
If each spell is a skill, it follows that skill points must be expended to learn a new spell (unless a spell can be used untrained). Other systems usually learn spells individually separate to the magic skill; some (Talislanta up to 3rd Ed) do charge XP for characters to learn spells, while other games (D&D) would involve just a probable Gold Piece cost. D&D usually requires a roll to learn spells (significant in AD&D, largely a formality in 3E); Palladium has a spell-learning table which sometimes gives 'half-learned' spells (50% effect).
Rolemaster is interesting in having “spell lists” which once learned level up with the character, unlocking new spells. Some systems do not have individual spells at all and characters need to adapt a basic effect on the spot (Talislanta 4E, and Amazing Engine IIRC), while Ars Magica has rules for both that (“spontaneous magic”) and predefined spells.

Freeform magic systems are somewhat interesting in that they allow more latitude for 'creative' players. In games which also have 'real physics' to some extent, freeform magic may allow players to abuse physics knowledge by applying it e.g. a Mage: the Ascension hermetic wizard player is better able to apply 'Forces' effects if they understand some of what could be done by manipulating heat, magnetism or electricity ('I deflect the bullets by siphoning electricity to generate a magnetic field' or 'I explode the shotgun by manipulating the gunpowder reaction').
Freeform magic systems tend to give less refined results than codified spells, maybe downpowering some classes of effects because inherent limitations can't be considered and countered during the design phase. For instance, consider a spell that 'creates sword of holy fire' vs. 'create ball energy' (both Prime effects in oWoD Mage). The rules note that the sword effect is 'single target' regardless of how many people you hit with it (a bonus) but what isn't considered is that it doesn't mention is that the 'holy sword' variation is actually at a disadvantage because a fireball goes off immediately, while the mage making a weapon has to spend a round casting it (and not hitting anyone). (likewise, 'speed' effects are disadvantaged by requiring an action, reflexive defensive effects require specific action rules for that, and writing a generic option for 'quickened spells' may well lead to this just being abused for fireballs).

Part of our system is that we use spell points, but we use 8 different power sources, and different spells need different types and combinations of sources.  So part of the cost is based on different types of spell points/spell skills.

More powerful magics often use more types of spell skills as well as higher point costs.

we also have a recovery skill and a success skill in each type.  We have some casters who have a few levels in fire, but as many levels in recovery and can get back their whole amount of fire points in a few hours.

The StarCluster variants with magic - Blood Games II/OHMAS/Outremer and Book of Jalan - generally use up the linked attribute for the skill. If the magic is based on STR, it will cost a point of STR to use it. I know it isn't the only example of this, too.

-clash

Quote from: LordVreeg;506532Part of our system is that we use spell points, but we use 8 different power sources, and different spells need different types and combinations of sources.  So part of the cost is based on different types of spell points/spell skills.

More powerful magics often use more types of spell skills as well as higher point costs.

we also have a recovery skill and a success skill in each type.  We have some casters who have a few levels in fire, but as many levels in recovery and can get back their whole amount of fire points in a few hours.

Thanks LordVreeg. I hadn't considered systems which have multiple sets of spell points at all. Come to think of that, I think Ars Magica may have a very distantly related idea (pawns of vis, or stored magical energy, often only work for a particular type of spell), and there are systems where you can multiclass and have different sets of resources powering different spell lists, but I hadn't seen that idea exactly before.
The variable recovery skill is a new one on me too.
Thanks!

Quote from: flyingmice;506587The StarCluster variants with magic - Blood Games II/OHMAS/Outremer and Book of Jalan - generally use up the linked attribute for the skill. If the magic is based on STR, it will cost a point of STR to use it. I know it isn't the only example of this, too.

-clash

Tunnels and Trolls which I noted in spell points is quite similar, actually . I bundled ability damage based spellcasting into Spell Points since its the same basic principle (x points, y cost) although perhaps I should break it out more..?
I love systems that do that, actually, since it adds considerations to spellcasting that can deter players just burning through all their points, and since characters don't get unbalanced extra value out of the spellcasting ability score- when you've burned your high STR up  for spells you lose all the usual bonuses for beating up people. Slight additional overhead in recalculating bonuses occasionally of course.

EDIT: have found there are a couple of OSR games (e.g. Microlite D20) which burn 'hit points' as spell points. Interesting way to make wizards squishier in games where everyone has the same HP. This approach does need safeguards to prevent magical healing loops (the original version of Advanced Fighting Fantasy has a problem where a Stamina spell costs stamina to cast - a wizard can heal back to almost full after every fight, the only problem being the chance of fumble).

Quote from: Bloody Stupid Johnson;506679Tunnels and Trolls which I noted in spell points is quite similar, actually . I bundled ability damage based spellcasting into Spell Points since its the same basic principle (x points, y cost) although perhaps I should break it out more..?
I love systems that do that, actually, since it adds considerations to spellcasting that can deter players just burning through all their points, and since characters don't get unbalanced extra value out of the spellcasting ability score- when you've burned your high STR up  for spells you lose all the usual bonuses for beating up people. Slight additional overhead in recalculating bonuses occasionally of course.

I missed the reference to T&T, but T&T IRC only uses STR, while the SC games use all the attributes. It forces spellcasters to cast different spells, using different attributes, to spread the loss around, and since the SC analog of HP is a multiple of the character's physical attributes, it damages the spellcaster directly as well.

-clash

Quote from: Bloody Stupid Johnson;506679Thanks LordVreeg. I hadn't considered systems which have multiple
sets of spell points at all. Come to think of that, I think Ars Magica may have a very distantly related idea (pawns of vis, or stored magical energy, often only work for a particular type of spell), and there are systems where you can multiclass and have different sets of resources powering different spell lists, but I hadn't seen that idea exactly before.
The variable recovery skill is a new one on me too.
Thanks!

welcome.
I try to run a truly mutant system...  
Not just a rehashed mix of other peeps rules.

Quote from: flyingmice;506687I missed the reference to T&T, but T&T IRC only uses STR, while the SC games use all the attributes. It forces spellcasters to cast different spells, using different attributes, to spread the loss around, and since the SC analog of HP is a multiple of the character's physical attributes, it damages the spellcaster directly as well.

-clash

Ah I see...so Outremor effectively has multiple spell point pools as well! Cool, thanks. Spreading the damage around should make min/maxing harder as well...

Quote from: LordVreeg;506689welcome.
I try to run a truly mutant system...  
Not just a rehashed mix of other peeps rules.

Mutant is good...