From 1d4chan
Acererak's remains as shown in module S1.

Acererak is the name E. Gary Gygax bestowed upon THE LICH (no, not that one) in Tomb of Horrors, technically a demilich as of module S1 1978. He's called "The Devourer" after his self-portrait in that module. *erRRp*

Bruce R. Cordell two decades later, in Return to the Tomb of Horrors, laid out the backstory (with Gary's blessing):

Acererak, cambion son of Tarnhem balor tanar'ri (sic: Return is 2e) and a human woman he'd raped, lived a long life doing dark arts. Somewhere in the Negative Material Plane, the man found Moil, "The City That Waits", which no less a horror than Orcus had ripped off planet Ranais. Later, nearing death himself, he chose to become a lich. He set up his own tomb, in that swamp which (later) separates the Scarlet Brotherhood and more civilised folk. This tomb is, in the remake, Acererak's gateway to Moil.

Acererak as a lich rediscovered Moil still floating around the Negative Material. The lich went there and recruited the local deadites to build his new, real tomb of horrors: the Fortress of Conclusion. Acererak is going for an Apotheosis - to pick up where Orcus left off. (Again: 2e. But late 2e. Orcus is here going by "Tenebrous"; he's handled in inferior work.) Meanwhile Acererak's filled the Fortress and Moil with traps partly because he's a dick like that; also because the higher levels adventurers attain, the more potent are their souls to fuel his Apotheosis.

Over the course of Return the heroes get to foil Acererak's plan - in fact, even to destroy his phylactery. But, wait for it . . .

In the third of 3.5's Tome of Magic that's actually good, Acererak returns as a Vestige for the Binder class.

Somehow the Devourer shows up alive, or undead?, again as the BBEG of Tomb of Annihilation. His new plan is to create a god he can control instead of becoming one himself using a "soulmonger" to suck up the souls of the dead all over the world and also slowly kill those who previously died and came back and use those souls to supercharge an aborted god fetus into a full god. *deep breath* In this adventure Acererak is depicted as a lich instead of a demilich. It is unclear if this is because he somehow came back to unlife as a lich again, or because this is actually a prequel to Tomb of Horrors, or because of WotC not caring about maintaining continuity (probably the last one considering how they treated the Ravenloft setting).