Reading through the RC, I'm a bit puzzled about the Immortals concept. how do they go together/what realtion do they have to other gods in the multiverse?
The planar layout looks a bit like it is incompatible to the AD&D Version. Although Demogorgon and Orcus are in the Immortals Supplement.

Strange.

Mystara in its RC incarnation wasn't a part of the rest of the "multiverse".  Later on it got turned into just another AD&D setting (and tanked soon afterwards) but at the time the RC came out Mystara was the world exclusively for the D&D "basic" rules, with its own planar geography and rules.

So, Immortals, not gods.

RPGPundit

The RC comes from the period when it was very important to TSR that moms be able to look inside a D&D book and not find gods or demons.

So it's nerfed. Immortals = Gods?
Like T'anari = Devils?

Quote from: SettembriniReading through the RC, I'm a bit puzzled about the Immortals concept. how do they go together/what realtion do they have to other gods in the multiverse?
The planar layout looks a bit like it is incompatible to the AD&D Version. Although Demogorgon and Orcus are in the Immortals Supplement.

I wasn't that heavily into it in Mystara and the basic game, but it was explicitly possible for mortals to become "immortals", and it's quite possible that all immortals were once mortals. There really was no separate god category.

What they did with this after they rolled the setting in with 2e, I don't know.

Quote from: SettembriniSo it's nerfed. Immortals = Gods?
Like T'anari = Devils?

For the Tanari sure, but in D&D the Immortals read differently to me. Immortals never really seemed deific to me, it was more like they were champions for their spheres of power which had a primordial and elemental slant to them. There was no real god of Matter, Energy, or Entropy, they were simply an incredible source of raw power that could elevate chosen mortals to an immortal state in order to spread its influence across the planes.

*double post*

Caesar Slaad and JamesV have summed  it up pretty well as far as the practical and conceptual differences and jrients is probably correct as to the origin of the terminology itself. Put all three posts together and there's your answer.

There was a certain element, no doubt, of orders from on high, Lorraine Williams was determined that TSR would bend over and be the Christian Right's bitch in a pathetic and pointless attempt to "sanitize" D&D, precisely when they should have been reveling in the status as something dangerous and rebellious (which is far better than its current image, as something dangerously pathetic and nerdy).

As for the Immortals though, if the designers of the D&D game were forced to use that title instead of "gods"; it ended up serving a very distinct purpose because D&D Immortals became truly different from AD&D Gods. Most principal of which is the clear fact that in D&D all of the Immortals were once mortal adventurers, and that there is a clear path to become Immortal in D&D.

RPGPundit

So, how can I bring those cosmologies together? Orcus and Demogorgon rule in their respective outer planes, so not all hope is lost.

Wasn't the Immortals stuff before the times of the Lady of Pain?

A mash-up of RCD&D immortals with the outer planes? I'll put something together, it might not work for you, but it could be a jumping off point.

I am so looking forward to it!

All right, this cosmology and brief mythology is based off the primary assumption that the everthing is based off the RCD&D interpretations of immortality, mixed with 3rd edition D&D planes and a possible explanation for religions. Each paragraph I will insert a parenthetical to denote what planes I'm talking about. For this part I'm thinking of the elemental planes, the prime material plane, the outer planes. I'm all up for discussion on its merits and flaws.

(Elemental) Before there was anything there were the Spheres of Power. Matter, Energy, Time, Thought, and Entropy were boundless powers driven to expand themselves, at the same time knowing that only one had primacy in the absence of balance, the weathering and decay of Entropy. In their drive to dominate, they first created a home for themselves. Each Sphere created a boundless space for themselves, surrounding themselves with a substance each felt comfortable with. Matter lay quietly with the unyielding earth. Energy burned with the heat and chaos of fire. Time sought the flows and eddies of water. Thought swirled and danced with air. Around them all, seeking to smother them yet unable to, waited Entropy. It was soon felt by them all that while a harmony was created, there was no means for either to push themselves outward, to expand, and so a new place created.

(Prime Material) All of the Spheres of Power came together to create this new place, they not only placed their own mark on this plane, but they mixed and blended together, creating a variety and tension where all could contest each other and seek to grow. Rivers flowed, mountains rose, winds blew, magma boiled, and all of them could lessen and wither. Soon they were no longer content with this seemingly passive contest, and created life as a means of giving their world a force outside of themselves. It was in this endeavor alone that all five Spheres joined giving all forms of life a body, mind, a sense of mission, wisdom, and the inevitability of death. Life thrived here from the moment they were created and even better for the Spheres, some of the forms of life became their advocates on the plane without their slightest inducement. Soon all of the Spheres granted powers to the greatest of their champions, making them stronger, weilders of potent mystic power, and more importantly, immune to the ravages of time. Violence was the only means to silence these powerful souls.

(The Outer Planes) For a long time the immortals were great champions for their Spheres on the prime plane, yet as eons and their struggle passed, their opionion toward their home changed. Their powers gave them great pleasure and reward, but they also felt both uncomfortable with the monotony of the homes of the Spheres and the now seemingly weak and colorless prime plane they warred over. They sought a means to create idealized homes for themselves, less boring and cold than the elements, more durable and comfortable than the prime. The immortals soon pooled their efforts and in time whole dimensions sprang into being and named them in turn: Bytopia, Limbo, Mechanus, Celestia, Gehenna. Some places were firmly in the power of one sphere's immortals, while others sought and found harmony between two or even three spheres. Soon these immortals reigned as lords of their planes, no longer direct champions for the Spheres on the prime plane they acted more as agents, encouraging their worship and by extension their Sphere amongst people, and by finding great heroes like themselves to do their will with the prize of immortality for those whose deeds were the greatest.

Okay, enough of me ruining the english language, how about I put that whole section into plain language.

In some ways I've turned the traditional cosmology on its head, Instead of just being solid balls of element, the elemental planes are enormous sentients, brimming with cosmic power, and swirling around it is Entropy, so what that does to the astral and ethereal planes is anyone's guess. Gods and demon lords still exist, but in a new fashion. These beings are now immortals, who use their powers to not just benefit themselves but their Sphere, which is the real source of their power. The outer planes are constructs of these immortals and can have their own unique but essential links to some of the Spheres.