She of the Thousand Young may have been summoned to Athas by the remants of the Aboleth still slumbering in their black lakes deep below the surface, or she may have spawned them too. The inscrutable Illithid have followed Abhoth to Athas, intending either to harness or destroy it for their trans-temporal schemes (originating from the final era before the Heat Death of the Universe, the time-hopping Illithid have a vast and maniacal scheme to attempt to counteract this cosmic event and so save themselves). Eldritch machines and arcane traps distinguish Illithid strongholds in the darkness below.
Cults of the Mother of Monsters (no true clerics, of course, but using sorcery and powers of the mind) secretly plague the surface and even managed to engineer the fall of Kalak. Tyr will soon be rife with the hedonistic fertility cult that ultimately leads to very horrific... liaisons... with the tentacled horrors from beneath. Even the corrupt and evil order of the Sorcerer Kings is opposed by these devotees of absolute chaos, in whose fevered dreams all of Athas is rendered down into a boiling muck of protoplasm.
You know, just to make Athas a little harsher.
If there is interest, the 3rd party publisher called "Goodman Games" sells a PDF bestiary of Lovecraftian monsters for 4E as part of its "Critter Cache" line. The Shoggoth is a Level 18 Elite Brute; the Greater Shoggoth is a Level 24 Solo Brute and clearly has no sense of boundaries.I think it's also a way to introduce an Underdark to Athas without it being a place that's actually more hospitable than the surface!
I've considered incorporating the Cthulhu Mythos into Dark Sun as well, I like the idea, but I think you're getting Abhoth mixed up with Shub-Niggurath.
Thoughts on incorporating typical deities and species:
Azathoth - Might be the being star pact warlocks ultimately get their power from.
Chaugner Faugn - Worshipped by a small tribe of halflings in the forest ridge, feared by neighboring tribes, but not really known of beyond that.
Cthugha - Easily a patron for fire priests, especially the less sane ones.
Cthulhu and his star spawn - Due to the lack of water, I'm not seeing Cthulhu himself being especially present, except perhaps in a very deep aquifer.
Dagon and the Deep ones - *gasp* *GASP!* Long gone. Their lost city is probably flooded with something like a combination of resin, garum, and fossil fuels, perhaps a new resource that would start a war between all the city states.
Hastur - A minor, messianic religion typically ignored by the sorcerer-kings. New adherents believe they will be purified and will act as purifiers for the world, but do not teach preserving magic (so the SKs go "meh, useless metaphysical gobble-dee****.") In reality, the upper priesthood are a cabal of defilers that believe the universe is fated to entropy.
Insects from Shaggai (Shan) - A minor cult of Azathoth develops without explanation. All psions, even the SKs, are confused by the minds of its followers, since they still appear to be frightened humans. Eventually, one SK, trying to solve the problem himself, probes too deeply into the mind of one cultist, opening his mind to one of the Shan. Although that one insect from Shaggai is easily destroyed, it manages to keep the SK's mind open just long enough for more to swarm in. After an hour long seizure by the SK, everyone in his palace lies dead or catatonic on the ground, and the SK stands and smiles, for the Shan now control a city state which they will use to perform bizarre and tortuous rites to Azathoth. (I'm thinking Draj).
Ithaqua - Assuming he hasn't died from the heat, it'd be an interesting change of pace for the characters to be travelling through the desert, set up camp in the mountains, and wake up to freezing weather. This would allow Gnoph-Kehs to survive, should the new hazard of freezing weather not be a tough enough challenge for the PCs.
Mi-Go - "Yes, we saw that lifeshaping technology your halfling ancestors had. Why don't you give it to us? You don't have it? Well, then we're just going to have to use you for experiments."
Nodens - Probably busy in some other plane hunting something really scary, because the SKs would be too much fun to hunt otherwise.
Nyarlathotep - Perhaps the patron of dark pact warlocks. Or hell, any pact. Also a way to incorporate magic items into the game: just give higher level item with some sort of monkey's-paw-like drawback.
Shoggoth - "Oh, look, an oasi-OHHAMANUHELPME!"
Shub-Niggurath - You've got a good idea, and Shub-Niggurath could actually be worshipped by primal characters as well. Her cult would be especially opposed to defilers, creating an opposition to the cult of Hastur not seen in other settings. The Oba of Gulg could (falsely) claim to be an aspect of Shub-Niggurath, though (not that Shub-Niggurath would care), to keep the populace in line and use the new cult's popularity to her advantage. Shub-Niggurath would restore the plant life of Athas... But would it be worth it?
Shudde M'ell - Easily a patron for druids and earth clerics.
Tsathoggua - Another possible patron for the dark pact. Probably just a little more powerful than the SKs, but thankfully lazier.
Yig - Worshipped by some Yuan-ti and the occasional crazed desert tribe (who want to sanctify the party by helping them shed their skin).
Yithians - "Oh yes, we saw that lifeshaping technology your halfling ancestors had. It amused us greatly. Here, have some bizarre technology (aka magic item) and some insanity."
Yog-Sothoth - Yog-Sothoth's cult would be small, and mainly consist of arcane characters, occasionally psionic characters. The SKs would probably persecute the cult, and the Veiled Alliance would be ambivilant to its members.
Good comment on Tsathoggua, too.
I've considered incorporating the Cthulhu Mythos into Dark Sun as well, I like the idea, but I think you're getting Abhoth mixed up with Shub-Niggurath.
Well, I don't follow "Chaosium Orthodoxy" or anything like that.
I'm actually more of a Clark Ashton Smith fan than a Lovecraft fan, and so I borrowed the name... you're right that what I put together is really a combination of Smith's Abhoth and later authors' conceptualizations of Lovecraft's Shub.Abhoth comes from Smith's Hyperborean story entitled "The Seven Geases". It is an absolute hoot and highly recommended.
I'm more of an REH fan myself (why the Cthulhu mythos seems appropriate for Athas to me), and then pan-mythos (except Derleth beyond "saying Haster will get you eaten by a Byakhee," and the distinction between the Great Old Ones and Elder Gods, although not the good-vs-evil aspect of that distinction, I prefer more of "local threat-vs-not-so-local-threat" deal).
I'm sure that Cthulhu can survive without water;). Anyway my thought on it (as I feel the mythos has a place in Dark Sun also), was one of the "abandoned ruins." But I wanted to populate it with Mind Flayers has his new worshipers.
I've also always figured that if I did anything with the Far Realm Cthulhu would be very present.