Kyuss

Kyuss, also known as the Worm that Walks, was an Elder evil prophesied to usher in the Age of Worms, a time when the undead will rise and destroy the world.

Description
The Worm that Walks was a 30 feet (9 meters) tall giant composed of a shifting mass of maggots and worms filled with the psychic imprint of a former demigod named Kyuss.

History
The true story of the being known as the Worm that Walks was a mystery to scholars. The only known fact all agreed on was that the Worm that Walks was originally a demigod called Kyuss, who had lived in the ancient past as a mortal necromancer and created many aberrant undead creatures, such as the Spawn of Kyuss that plagued the Underdark.

Eventually, Kyuss wickedness made his contemporaries to exile him to the fringes of society. Rather than languishing as an outcast, he became the prophet of a wicked cult who believed in the dreaded Age of Worms. During this time Kyuss discovered the ruins of a dead civilization in an island near Maztica, known as the Wormcrawl Island by the elves of Evermeet, and claimed the site as its own, where he ruled as a god-king. In the ruins, his followers discovered ancient metal plates, which held terrible secrets but also confirmed every prophesy he had spoken.

Through these inscriptions, Kyuss discovered how to transcend mortality at a terrible price—the lives of his congregation. On one bloody night he instructed his priests to slaughter them all, and after that Kyuss murdered the priests as well. When he was finished, he fused his essence with the foul ruins, gaining and losing his divinity in one moment. He was neither mortal nor god, and he was bound forever to the ruins. However, he was transformed into something far worse.

Some scholars believed Kyuss was related to the Far Realm-infested star Nihal. Either Kyuss was its brother or both were the same being observed from two different points in time.

After the Time of Troubles, a coalition of banites, bhaalites and myrkulites uncovered ancient scrolls regarding to Kyuss, and tried to find Wormcrawl Island in hopes Kyuss would resurrect their respective gods.

Appendix
The Age of Worms adventure path describes a possible backstory for Kyuss on the Realms. However, the campaign that it was written for is noted as being non-canonical in the Realms.

Non-Canonical Background
According to legend Kyuss was an extremely powerful tabaxi and a charismatic prophet in Mezro who led his cult out of that city in -653 DR after persecution from the mainstream followers of Ubtao. He wanted to become an obscure deity of the Chultan Peninsula.

Kyuss and his followers founded the religious city of Kuluth-Mar in the Chultengar. His cultists discovered ancient metal plates that foretold the future and confirmed every prophecy that Kyuss had ever made. Kyuss was personally visited by his god, Jergal, who gave him a gift of a single green worm and tutored the man in the ways of necromancy

He began experimenting on the undead, creating such horrors as the Kyuss worm, the spawn of Kyuss and the avolakia. At this time, he gained the services of the red dragon, Dragotha, who he transformed into a dracolich to serve as his general of his undead host. He created a cult around the dogma of life being only temporary, the afterlife a lie and undeath being the one true path to immortality.

While his cultists grew ever more devoted by the apparent truth of his visions, Kyuss tried to fulfill the greatest of his prophecies. He sacrificed powerful magic items and built a massive citadel, the Spire of Long Shadows. At the top of the Spire, he created a monolith through which he channeled the terror, adulation and souls of his entire cult and achieved apotheosis.

However, an alliance of couatls, eladrin wild dwarves and druids trapped him in the monolith, which prevented him from ascending to true godhood and relegated him to a historical footnote.

Two millennia later, a coalition of banites, Bhaalites and Myrkulites known as the Ebon Triad who sought to resurrect their dead gods, initiated a plan to free Kyuss and use his divine spark to achieve their objectives. . In reality, the Triad was created by Kyuss's high priest, the silver dragon vampire Lashonna, who is utterly devoted to him to the point of love, and an avolakia who in its disguised form serves as the Triad's leader.