Don't get all excited. I don't know when I'll be ready to run, and this is the same megadungeon I noted back in January. Since I'd almost assuredly open it to FLAILSNAILS, that means I have to consider Clerics and Magic-Users that might arrive from other games.
Is it absolutely necessary to change the magic rules? Probably not. However, I would like to highlight the weirdness that is Carcosa, and accentuate the fact that native Carcosans are probably the best equipped to handle Carcosa (although I really like the persistent idea that some planeswalker will arrive and start organizing the populace like John Carter of Mars or Lawrence of Arabia).
Here's the thing, though. To tackle this subject, we have to consider D&D magic — after all, Crusssdaddy already wrote a post about this on the Doomed World CARCOSA (and one for magic-users), so why am I doing it again?
Simple. I really like the concept, but the magic works differently in my head. Here's where I'm coming from.
(Also, as an aside, I might not keep these rules, but here's my current thought.)
Clerics are fanatics and faithful who pray to the divine and are rewarded for their efforts. That's really simplistic, but the bottom line is that clerics devote themselves to the divine, and so the divine devotes itself to clerics. Whatever the particulars (some clerics pray to multiple deities while others only revere one), there's a give-and-take relationship present.
Clerics may not specifically pray for spells, but it's obvious their devotion grants them a pipeline to the gods. In order for their magic to work, they have to maintain that open channel.
Carcosa interferes with this in two ways.
First, as noted in the Doomed World CARCOSA post, "The Old Ones dominate CARCOSA and the other Gods of the multiverse aren’t too keen on intruding." I agree, although I'd also point out that Carcosa has a limited connection with the spirit world, suggesting the people are more familiar with corporeal things. I'm inclined to keep the divine spell failure rules posited in that blog post: "Cleric spells have a base 50% chance of failure, modified downward by 5% per level of the caster (ex. a 3rd level Cleric has a 35% chance of failure, an 8th level Cleric has a 10% chance of failure, at 10th level and above the chance of failure is removed). A failed spell counts as spent."
Second, as noted, "Praying for spells can be dangerous and calls to distant Gods are prone to attract unwelcome attention." However, the some of the rules and comments in the article seem to imply that the Old Ones are divine and hold dominion over the gods. In my head, as in Call of Cthulhu, they're not. They're physical entities, and for all his power, Great Cthulhu with 57 HD is just a variation (albeit a weird variation) on a Fighting Man around level 57. Power level has nothing to do with it; although the world's greatest boxer might be able to physically defeat the Grey God of Lightning and Tailors, one's a god and the other is not (although a Fighting Man at 57th level is hardly just a normal human anymore, either, he's not divine).
As such, casting cleric spells doesn't risk drawing the Old Ones' wrath. However, it does draw their notice: a cleric must save vs. spells/magic at +2 or else have the expenditure of power be noticed by one of the Old Ones. Within 1-10 days, the servitors of one of the Old Ones will arrive to investigate and likely kill the cleric. Which Great Old One notices the expenditure of divine power (and subsequently sends servitors) is up to the DM, who is free to use the d10 chart on the above blog post. Common sense may also apply — if you're in the middle of the desert, Cthulhu probably isn't going to send deep ones after you.
Unlike Crusssdaddy's post, I assume that turning only works on undead, and that spells tend to work the same, albeit with some Carcosa trappings. Clerics seen casting spells are considered to be sorcerers, mutants, or stranger things, and so will be treated accordingly.
Arcane Vancian magic is a strange animal, typically abstracted in D&D and its incarnations. As D&D is implicitly post-apocalyptic (and some settings imply magic is modular and almost technological in its origin; see this Jeff's Gameblog post on sufficiently advanced magic for an example), magic is typically seen as a lost art. Its heyday has passed, and only shadows of once-great power remain — Chaucer even writes about long-gone days of magic in The Wife of Bath's Tale, saying, "In th' olde dayes of the kyng arthour, / Of which that britons speken greet honour, / Al was this land fulfild of fayerye. / The elf-queene, with hir joly compaignye, / Daunced ful ofte in many a grene mede. / This was the olde opinion, as I rede; / I speke of manye hundred yeres ago. / But now kan no man se none elves mo."
-C at Hack & Slash recently tackled Vancian magic in his post, "On a Change in View." As noted in the post, the best summation of Vancian magic is, "The design of this mosaic we cannot surmise; our knowledge is didactic, empirical, arbitrary." Vancian magic is vague because even the wizards don't understand it — they merely understand how to access it.
But what if Vancian magic is vague not because it's dense but because it's unsafe? Carcosa, pg. 111 states, "At the height of their powers, the Snake-Men destroyed themselves by releasing ultratelluric forces impossible to control." What if that powerful ancient magic from the histories messes with the same fundamental forces as the Snake-Men's sorcery? What if civilizations with powerful magic are rare (almost every D&D setting shows magic in decline) because they tend to destroy themselves — see the Doomsday argument and Fermi Paradox for real-world theories regarding the same problem.
Whatever the case, it is assumed that arcane magic taps into the fundamental forces of the cosmos without divine intercession. The magus is the intercessor, and he has to deal whatever happens.
In Carcosa, that means he has to deal with the leftover energy from the Snake-Men's disaster. Those ultratelluric forces are still available, and wreak havoc on the fabric of magic. Unlike clerics, who become more adept at contacting their patrons from anywhere in the multiverse, this never gets easier for magic-users. Low-level magic-users cannot adequately prevent backlash, although they tend to throw less power around. High-level magic-users become much better at preventing backlash, but channel more magical power. The problems of control and efficiency effectively cancel each other out.
As such, whenever a magic-user casts an arcane spell, the player should roll a d20. Nothing happens most of the time, but if he rolls a "1," the caster is struck by a backlash of cosmic force. The DM should roll 1d10 and consult the following table:
1 - The magic-user is subject to unnatural aging, aging from 1-5 years instantly (roll on the Unnatural Aging chart).
2 - The magic-user takes 1 die of damage.
3 - The magic-user gains a random mutation.
4 - The spell fails, and the magic-user can cast no more spells that day.
5 - The magic-user acts randomly, as if affected by confusion.
6 - The magic-user goes insane as if struck by an insanity weapon.
7 - The magic-user loses a level, to a minimum of 0.
8 - The magic-user takes a number of dice of damage equal to the level of the spell.
9 - The magic-user summons a random creature. The creature appears within the caster's line of sight, almost as if summoned by a summon monster spell. This creature is not predisposed to like the caster and will likely attack. The DM should determine such a creature by either rolling on a local encounter table or by generating a random Spawn of Shub-Niggurath.
10 - The spell goes awry, with twisted effects: attack spells hit allies, or even the caster; healing spells deal damage; protection spells grant vulnerabilities; and so forth.
Astute observers will note that incurring backlash does not necessarily cause the spell to fail. Even if backlash kills the caster, he will cast the spell unless the particular backlash in question contradicts this claim.
Conversely, if the magic-user rolls a "20," something unexpected (but positive) should occur. Roll 1d10 and consult the following table:
1-4 - The magic-user does not expend a spell slot by casting the spell.
5-7 - The spell's variable effects are maximized.
8-9 - The spell's effects (range, duration, damage, etc.) are doubled.
10 - Instead of the spell the magic-user intended to cast, the magic-user casts a random spell from the magic-user spell list. This spell is always appropriately targeted — attacks hit enemies and protection spells target friends — but can otherwise be any spell from the list. Yes, your Level 1 Magic-User might cast wish, although that's probably just more trouble than it's worth.