Kyuss is an eldritch god-like abomination from the fictional universe of the Forgotten Realms pencil-and-paper roleplaying game.
Kyuss is one of the Elder Evils, known as the Worm that Walks. It is prophesied that Kyuss will bring forth the Age of Worms, when the undead will rise and destroy Toril. His form is that of a 30f tall humanoid eldritch abomination consisting of a continuously shifting mass of worms and maggots. Whom carry the psychic imprint of Kyuss, a former demigod.
Kyuss was originally a mortal necromancer who lived in the distant past, notorious for his aberrant undead. He was eventually exiled by his contemporaries and became the leader of a dark cult, which believed in the Age of Worms. Discovering the ruins of a dead civilization on an island, he made it is known and ruled over his followers as a god-king.
His followers would discover ancient metal plates with inscriptions that contained terrible secrets, confirming that what Kyuss prophesied was true. Using the inscriptions, he performed a bloody ritual. His priests slaughtered all of his followers, after which they were in turn murdered by Kyuss. Afterwards fusing his lifeforce to the ruins, at which point he both gained and lost his divinity. Becoming neither a god nor a mortal, Kyuss was now bound for eternity to the ruins and in the process transformed into something worse.