Here's a great review of Demon Cults 3: The Cult of Selket, it sounds like this could fit right into Dark Sun with little or no adjustments. I think I'm going to have to pick this up!
In the third volume of the Demon Cults series, we learn of the cult of Selket, a deity revered by many desert-dwellers. With the scorpion as her sacred creature, she is a goddess of healing, death and the afterlife. Desert-dwellers look to her for protection against dangers such as venomous creatures, sandstorms, and the blazing sun, seeking her healing power for their afflictions, and her vengeance on their enemies.
All this sounds like a perfectly reasonable religion... if it wasn't for the way in which devotees, organised in cell structures terrorist-style, hang out around oases and the outskirts of towns on desert fringes and seek converts through fear rather than through persuasion and love. Worship in hidden temples involves music, narcotic vapours and the handling of live scorpions... the faithful, should they survive, see it as a mark of divine favour. Somewhere deep in the desert lies the ancient and ruined City of Scorpions, which devotees want to see restored to its former glory.
The cult maintains a group of assassins, called the Desert Scorpions, who kill on command of the priests of Selket (and not for gain, like most professional assassins). Their leader, a dwarf called Sadiki Sefu, is presented with complete background and stat block, as is the Chief Pristess, Dakhamunza Sat Selket, Daughter of Selket. There's also full details of the guardian of the City of Skorpions, about whom I shall say no more in case your party decides to go there!
These notes are followed by a selection of adventure ideas, organised by APL, which can be used as one-offs or in a campaign arc in which the party defends the land and whatever faiths they hold to be true against the cult. Should you use the Midgard campaign setting, there's a sidebar explaining where the cult fits in; whilst for those who'd prefer to embrace the worship of Selket there are notes on 'playing for the other team'! With a purview of healing, death and the proper passage into the afterlife, it's quite plausible that a party might decide to help Selket's devotees to restore her worship rather than fight against them.
Next is a new materials section, with new monsters (venomous mummies for starters...), a new magic item and a spell, and finally a couple of new traits: Selket's Favour (remember the scorpion-handling - you're good at it) and Expert Embalmer, the skill of mummification.
There's not really anything demonic here, it's a solid desert cult that should fit into whatever deserts there are in your campaign world nicely, with options to use them as allies or adversaries, a neat touch.