Acererak is the name E. Gary Gygax bestowed upon THE LICH (no, not that one) in S1: Tomb of Horrors, technically a demilich in that module. 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, the cambion son of a 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 multiverse (retconed as part of the Ethereal Plane), the man found Moil "The City That Waits" - which was no less a horror than Orcus had ripped it off the planet Ranais - now floating in its very own demiplane, gravitationally "above" the Negative Material. Later Acererak, nearing death himself, 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.
As a lich, Acererak rediscovered Moil still floating around the multiverse, so he went there and recruited the local deadites to build his new, real tomb of horrors: the Fortress of Conclusion.
Along the way the party learns 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.) This would merge his consciousness with the negative energy plane and thereby become the negative energy plane. Yes, that dimension/plane which is the source and power for all undead. Let this sink in: he'd control all undead across the multiverse.
As an aside you'd think that might annoy all the other factions involved in undead; not least his former mentor/bestie Vecna, who is concurrently plotting his own route to godhood although, good Lord, not nearly as ambitious. (Maybe he can blow off Not-Orcus-In-This-Edition.) Want an alternative to the ending we got? Refigure that conclusion to GETTING THE FUCK OUT OF THE FORTRESS OF CONCLUSION so that the party cleric can commune with... anybody. Saint Cuthbert's dog if s/he has to.
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. Here the heroes get to foil Acererak's plan - in fact, even to destroy his phylactery. To much rejoicing by literally everybody else ever. But, wait for it . . .
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).