"Wretches! Death is a mere annoyance for us, but for you it is a new beginning!" - Acererak
Acererak ruled a kingdom in what is now Sunndi on the world of Oerth, and the center of his power was the Vast Swamp. "The name was ancient when eastern Oerik was still ruled by the Flan peoples, and the frightening tales of the Tomb of Horrors had long been a part of the folklore throughout the Flanaess when the Kingdom of Aerdy was but an idea posed by an Oeridian chieftain."
Apparently an ancient conjurer summoned a demon that was far beyond his ability to control. The demon devoured the wizard, and forcibly conceived a cambion son in one of the wizard's servant girls: Acererak.
Acererak's mother survived her son's birth, but she was killed by a torch-wielding mob ten years later. The boy was rescued by none other than Vecna, the Whispered One, who killed the advisors that urged him to kill the child and instead took the half-demon on as an apprentice. Even then, ten year-old Acererak loathed life, looking forward to becoming undead like his master.
"This life is nothing but a pain-filled maze, spaced with confusing puzzles that all lead to death in the end!" - Acererak
While alive, Acererak built a subterranean temple complex in the name of Orcus, burying its architect and all of its workers within. Eventually, Acererak succumbed to the lure of lichcraft, and had himself buried in a labyrinthine tomb where he committed himself to his studies and, eventually, demilichdom, abandoning his body for the planes beyond.
The demilich found the demiplane of Moil, the City that Waits. It had once been the capital of a civilization that worshipped Orcus; when the people began to turn to less-bloodthirsty powers, Orcus tore the city from the face of the world and cast them in a demiplane where they would sleep forever in a land with no sun. It is said that on that day, all children in Ranais lost the ability to speak, saying only one word, over and over again: Orcus.
Centuries later, Acererak the Devourer rediscovered the city, transforming the city's sleeping dead into an undead workforce used to create a fortress for their new master. Most of the inhabitants of Moil were destroyed in this effort.
Within his new fortress, at the edge of the Negative+Energy+Plane, Acererak began a necromantic Great Work designed to make himself and the plane a single, sentient, nigh-omnipotent entity. And the terrible energies released in this process, combining with the last, impassioned prayer of Quah-Namog, reached even the Astral Plane, where the corpse of Orcus began to stir.
That process, however, was interrupted by adventurers, who struck down Acererak and destroyed his phylactery. Since he was associated mostly closely with the Negative+Energy+Plane but souls do not dwell there, he became a Vestige.