The Great Cataclysm which destroyed much of the surface of Xhuul in the distant past was said to be caused by a war among strange alien gods and their servants. Ever since that great war, the many generations of descendents of the original surivors of that conflict have been resistant to put their faith in the very beings which caused such destruction. Most if not all of these gods have been forgotten, but the ancestral madness that drove such men to seek the favor of these alien beings has also survived these many generations. In these few men, the formation of cults dedicated to these foul, uncaring alien gods have arisen.
Cults are generally tolerated, their followers are few, and none have any true temporal power in the day-to-day lives of most Xhuulians. At least most people would hope they do not. In some remote regions, this may not be the case, where a village or town may be the nexus of a powerful cult, secretly trying to undermine or infiltrate their neighbors. Currying the favor of these alien god-like beings usually involve human sacrifice and foul acts of consumating with strange entities and other bizarre rites. The number of cults are too many to list, as most are secretive and do not announce themselves openly.
Sample Cults of Xhuul
KAKATAL, The Burning Chaos:
Appears as a massive ball of churning flame, with tendrils of flame whipping about. This being embodies the aspect of destruction, purification and creation. Armorsmiths, Weaponcrafters and any who work near a forge or furnace give homage to Kakatal, to bless their creations. Devout worshippers are said to be granted immunity from heat and flame.
PYARAY, Lord of the Ocean Abysses:
Appears as a huge blood-red giant octopus with a pulsing blue gem fixed atop it's head. This being commands the Chaos Fleet, a flotilla of sunken ships manned by undead sailors. The cult is strong in settlements near the ocean. Most sailors give lip service to Pyaray, making sacrifices of gold and gems in exchange for good weather on their maritime journeys. Worshippers are said to be granted the ability to breathe underwater or summon the undead.
KORMOG, The Waste Stalker:
Appears as giant, grotesque humanoid with large glowing eyes. It's large lumbering frame looks emaciated, and it's hunger knows no bounds. Kormog is a cannibalistic being that stalks the frozen wastes, hunting down and carrying off travellers to devour. The cult is small, usually located in the high, snow-bound mountains of Xhuul. Tribes of degenerate barbarians usually leave sacrifices for Kormog. Kormog is worshipped by the Yeti and the Gnophkeh. Worshippers are usually granted the ability to track by scent and gain resistance to extreme cold.
AYAI, The All-Knowing:
Appears as a large glass and metal monolith, glowing with points of light. The cult is small and is followed by humans who claim to have "heard the call" of their god. Each cultist is grafted with a device near the base of their skull to better commune with Ayai. Ayai is a god of knowledge and keeper of secrets. The cultists are usually found in ruins of the ancients, looking for sacred objects that Ayai has told them to find. Worshippers are granted bonuses to figure out ancient artifacts.
Appears as a writhing mass of flesh, sprouting, reforming and absorbing many appendages, eyes, mouths and other unnamemable organs. This is it's natural form, but it can take the form of any creature for as long as it wishes. Known to be a cult followed only by mutants with grotesque deformities, they hope to gain the favor of their patron god to be cured of their defects or gain the power of shape-changing so as to hide them. This cult is also followed by dopplegangers as well as human assassins that would desire the secret of shapechanging.
YIG, Father of Serpents:
Appears as a large scaly strong man with a serpent-like head. He may be accompanied by a swarm of writhing snakes. Yig is also worshipped by the Serpent People and their kin (Yuan-ti). Worshippers usually gain some immunity from poisonous snakes and ability to talk to snakes.
There are no Clerics or Druids in the Xhuul setting. This means there is no access to healing spells or ressurection! The "gods" of these cults do not grant their worshippers spells through a "divine magic source" as in Castles & Crusades. The "High Priests" of the various cults are not Clerics or Druids either. They are usually Sorcerers who have learned a few spells and seek greater knowledge from the beings they worship, which know spells equivalent to a high-level Sorcerer. Some High Priests may just be normal men, their insanity and fanatiscm being their only true power.
In many cases, the "god" followed by a cult only exists in the minds of it's demented followers. So not all cults have a powerful being behind it. There are some that do, but are uncaring of their followers and never manifest before them.