So, I am working on a new-ish setting idea, that include the idea that magic is an inherently dark, chaotic and ultimately corrupting force. I know, a completely new idea, that nobody has ever heard of before. However, I want these supernatural forces on the one hand  to be desirable enough to be enticing for the players, which means they need to bring a decent amount of usefulness and power. And on the other hand,  want them to be ambivalent and enough that the players might have constraints to use these powers. Effectively, I want to use the player's conscience as a major limit for the use of magic, a balancing factor if you will. As a token tribute to the laws of thermodynamics, destructive spells will also a lot simpler and easier to master than productive or healing spells.

Logically, the more powerful a spell is, the more sinister it also needs to be. Smaller utility spells aren't particularly harmful for anyone, but also quite limited (they are an entry drug, of course). Moderate spells usually require sacrifices and blood (as a consequence, it will be a bit easier and more common to enchant items than to cast spells directly). While the most powerful spells inevitably require human sacrifices or body parts. Due to this logic, the most versatile applications of magic are likely going to be summoning rituals.

Now, I am looking for inspirational material, ideas for sufficiently sinister spells, fitting sacrifices and so on. (The system is going to be a slightly reworked version of runequest/legend, but that isn't particularly relevant).
Are there any representations of magic as a black art that impressed you? What kind of spells and rituals would you expect in this context?

Ones dependent upon zero-sum fortune probability and deferred misfortune.

To harm people you shift misfortune and pain randomly among your moieties' balance sheet, to allow limited targeted directing of a range of maladies upon the desired victim and its moieties. To harm Bob you shift a specific pain or misfortune onto him from his circle of touched lives, in return someone in your genuinely loved circle of family and friends has to suffer an equivalent loss. Sort of like a double entry bookkeeping of karma upon a social network scale.

To gain something is truly hard this way because the powers granting this shifting are not in it for the altruism, they feast on the pain. Doing specific magic good for others so that random good comes back for those you care about pays no "misery interest" to the Gods of Dark Debt. Also, being a detached lone wolf is a fast way to strangulate off your power. The smaller your social network and the less dedicated your love, the less threads and weaker the bond of attachment, thus the dark debt has less targets and power to draw from.

Also being a lone wolf does not protect you from such magic. With less people connected to, and in less powerful ways, then the energies harnessed against you and yours (moiety) will have less targets to distribute the harm. Basically this ensures a deeply connected social life to both a) stay safe from magical harm, and b) be capable to inflict enough retaliatory magical harm.

Quote from: Beagle;843290Effectively, I want to use the player's conscience as a major limit for the use of magic, a balancing factor if you will.

That isn't known to work all that well;).

When the spells are cast, they weaken the barriers between worlds. Therefore, spellcasters tend to be haunted by spirits that are attracted to the energy of the spells who hope to use the weakened barrier between worlds to finally move on from here.

Have magic be powered by "life force" (hit points/whatever), with more powerful spells causing more personal damage, either to the caster or "volunteers". If a relatively difficult foe can be defeated by casting a spell(s) that use, say, 50% of the party's HP instead of a lengthy and possibly losing fight, they'd probably go for it... this time. Make willing participant's "mana" more potent than that from unwilling participants if you want sacrifices to be an option but maybe one less chosen.

Once ran a short but popular campaign that was a kind of WH40k-ish fantasy setting (i.e. much more overtly powerful and regimented than WFRP) where the players took part in the defense of a hilltop castle that was under assault from skeletal giant Golems (i.e. giants made from hundreds of smaller skeletons, they'd break apart when they took enough damage or reached the walls to begin their assault) and a massive black dragon "general". Spells were overpowered by using the entire life force of low-to mid-level sorcerers who would cast turbo-charged versions of things like Magic Missile and be burned to ashes in the process. A huge price to pay, but a necessary (and willing) sacrifice to defeat a far worse foe (in keeping with the WH40k aesthetic). That was probably 10 years ago and still gets talked about wistfully from time to time, might need to roll it out again some day.

Another option to reflect the nature of magic is to have the power be drawn directly from the environment around the caster. In a certain range/radius, people grow weary, maybe get a gray hair or two. Plants wilt slightly, food loses it's flavor, animals either sicken or flee. Depending on the power / duration / frequency of casting in certain areas, it could eventually become a blighted area where magic might not even work anymore until it has time to "heal". There is precedence for these type of effects in folklore from around the world, as warnings that foul magic is afoot.... and this might be a more palatable option than sacrifices.

We put out a game last year that features corrupting magic. If you scroll down to the Book of the Archon, that is a free PDF of new spells for the game, some of those may be helpful: http://www.bedrockgames.net/untitled1.html

From real-world magic, curses that equally curse the curser.

Healing magic that has risks, side-effects, and cumulative long-term health issues, and/or addictive properties. Makes the decision to heal now have a long-term consequence.

Juicy deep random mutation/side-effect tables, with little or nothing at the low end, and horrible, dangerous, permanent, and otherwise bad colorful effects at the deep end, and then a nice formula of modifiers based on things like how frequent one uses magic, how strong the effect is, the nature of the effect, and a cumulative factor per person. I've used this type of thing variously in different games for magic items (you need to will them on, and each one has a breakdown number with deteriorates with heavy use and/or bad rolls on the breakdown table), healing spells, and casting spells. It pits the players' greed for powerful effects against their fear of bad effects and of things happening to their magic inventory and their character, which can nicely mean they choose carefully and don't over-do the magic use.

You can have magic spirits who actually do all the magic, but who are actual characters, perhaps usually invisible, who have their own motivations and quirks. The more powerful ones may tend to stick around and make life unpleasant in various ways. This is like the classic Christian "devil's bargain" but can be given all sorts of flavors.

Or there may just be quite a few magical beings of whatever nature, who are attracted to magic and spells, and so the more you use magic, the more they're liable to show up. Magic use may also make them stronger, or bring more of them to the world in general, or weaken the seal on one or more portals to where they come from. They may come in various flavors with various powers and motives.

Magic items may work because they have magic spirits bound up in them doing the work. Eventually the binding will wear out and the spirit released...

You can also have a system where spells essentially all have permanent effects and that keep doing stuff. You may be able to get an intended effect to begin with, but it doesn't stop (or at least it takes time for things to settle down, longer for more powerful spells), and may be unpredictable what continues to happen as after-effects of any spell.

Another system is requiring actual significant ingredients for powerful spells, such that getting and consuming the ingredients will tend to cause all sorts of problems. Got any virgin prince hearts on hand? Dragon eggs? Large diamonds for smashing? Finger of someone who loved you?

As for the moral conscience thing and the concern some players may not care: Blasting someone with magic (especially, killing them, or eventually, once a magic-tormented person dies) may result in a haunting spirit, particularly for those who have no conscience about what they did. The ghosts might not rest until the perpetrator regrets what he did to them.

You might want to look at the (free) Zenobia RPG or the (not so free, but better iteration of the same rules IMO) 43 AD RPG.
And then, of course, there is Sorcerer where the only possible magic is summoning, and manifesting any kind of magic ability usually means you've got a demon doing it for you, and omghesmadenoughtosummonademon.
But for truly insane magic, look at adepts in Unknown Armies, and I'm not using the word insane lightly, here.

Take existing spells and darken them up a bit. For example:

The "web" from a Web spell is acidic and burns away any clothing a victim is wearing. May also scar the flesh of the victim.

A heal spell puts a permanent mark upon the injured person, a small tattoo of the god's symbol. How many of those would it take before they become noticeable?

Fireball spells that slowly turn the caster bald.

Waterbreathing spells that give the caster a permanent fishy stench.

A knock spell that not only opens the door but blows the caster back 20 yards.

I had a player once who ran a cleric which took a familiar. What the player didn't know was that the familiar was a small demon who, at the beginning of combat, opened a small portal to Hell. When a soul, good or bad, died it was immediately sucked into Hell. The PC's god was no longer getting souls to judge which was making him angry at the PC. The PC couldn't figure out why his god was angry at him.

Power corrupts, you know.

Check out the Last Gasp blog's writeups of Maleficars (wizards) and Mystics (clerics). He's got some simple systems/tables set up to pump more horror and weirdness into spellcasting. He's created some free PDFs for download in the 'downloads' area.
I've been using his ideas in our LotFP games and enjoying extra dose of horror they provide.

I also like the idea of 'living spells' that sometimes hang around after a casting... haunting the caster, his target or the area the spell was cast in. A sort of residual magic radiation that can bring on complications.
One of the newer Stephen King books, Revival, is about a faith healer (actually a mad scientist) whose healing powers have some nasty side effects.

Beagle, if I understand you correctly, you may find any edition of Call of Cthulhu's magic system will be useful.  Magic is rare, dangerous, and costly in attributes, sanity, etc.  Some payments are even permanent.  You should get a sufficiently creepy feel to the magic if you look at the spell names and read a half dozen.   Good luck.

Good luck at it.  My observations of several attempts at this over the years are almost invariably one or more of the following:

1) If the results involve a high chance § of permanent turmoil or disaster for those around the wizard, but doesn't directly affect the wizard, the player doesn't give a damn or the character gets off on it.

2) If the results involve a high chance of permanent turmoil or disaster for the wizard, the player figures it's a throwaway character anyway.

3) If either #1 or #2 is the case and the player is a problem solver-type, he won't touch the system because it gets in the way of getting the job done.

4) If either #1 or #2 is the case and the player is a hardcore RPer invested in long-term characters, he won't touch the system for obvious reasons.

Don't know why this is the case, but that's my experience with attempts at such systems/subsystems.

§ - "High" seems to be a pretty low threshold with this sort of thing, and I've seen 30% twig people out.

Soloman Kane had a necromancer who cast a spell that animated his chopped of hand to hunt down and choke his enemies one by one. (Which may also show up in a hammer horror film now I think of it).

Pretty powerful, but you can't see people casting it a lot.

For now, I have this idea of three tiers of spells, not so much depending on power, but on the need and severity of sacrifices (ranging from no sacrifices to animal sacrifices to human ones, of course).

Simple, not particularly exiting spells are mostly unproblematic and supposed to address everyday problems; they are also common enough that the line between wizard and non-wizard is blurry:
Some shepherds know how to draw runes to drive wolves away from their herds. Waterwitches can dowse for water. The old ground keeper from the orchard knows that if he bury the afterbirth of a cow between the old apple trees, the harvest will be more plentiful.  There actually are spells in this tier that are more relevant for adventurers (spells that accelerate healing, protect against minor misfortunes, that sort of thing) but for the most part, this is harmless, convenient magic. There are curses of this degree of power, but they are more inconvenient or embarrassing than actually harmful: causing baldness, warts, turning milk sour, that sort of thing. It is also considered to be illegal and even immoral, but as long as the practitioners of these oh so evil arts are familiar faces and not too inconvenient and the harvest was good enough that scapegoats aren't needed, the neighbours won't care (and will be secretly quite happy to have someone in their midst who can heal their cows or drive off hungry wolves if the winter is long and harsh). However: outsiders, thugs, homeless strangers, vagabonds and other scum will be persecuted heavily (and punished severely and supposedly unfairly) if the rumors of witchcraft prove to be true.
Considering the fact that traveling adventurers sometimes look quite a lot like vagabonds and so on, it is to be expected that the players will feel the righteous wrath of the oppressed (especially because they are probably going to use what magic they can learn. They are adventuring PCs, after all, and they just know will need that extra edge sometimes).

On the next higher level of power, there are spells for the most part less harmless. Despite being more powerful, these spells are not necessarily more convenient (after all, if you need to sacrifice a lamb for a spell, you need to have access to live sheep), but if you are going to be convicted anyway, they might be worth it: with these spells, you can turn into an animal, create floating sheep eyes that spy for you, cause fear or lust in others and so on (the whole thing wouldn't be sufficiently creepy without occasional sexual connotations; besides using eldritch powers one barely understands for the sake of getting laid seems incredibly shortsighted and therefore human to me). Curses on this level are actually brutal, and dangerous – atrophying limbs, blindness, that sort of thing.
These spells include some which are very helpful, but usually with a twist: You cannot simply cure a disease, but you can transfer it from the patient to a live pig (which should then be killed and burnt, before it infects its fellows. This is the level of magic, that uses eye of newt and poultices made out of earthworms and while there are still a few whimsy spells here and there, the whole matter is more serious. This involvement in the magic arts is of course, completely scandalous and would be sufficient to shock the good neighbours, ruin reputations and may even ignite riots (after all, if you can cure a sick farmer by infecting a pig, you probably can cure your pigs by sickening the farmers).

The highest level of spells are, of course the reason why magic (or witchcraft, I'll use these two terms synonymously for this setting) is forbidden and loathed, mostly because these spells require human sacrifices.  The ointment used to create flying spells is made from human fat (according to the malleus maleficarum). These spells could probably just kill people, but that would be a bit anticlimactic from a game perspective. And this is also the level where I still need some neat

Well, not so sinister really, but DCC wizards get more and more fucked-up by corruption as they go along.