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 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. (In this part in Orcus' career, that other undead god is 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. Although later canon will say nahh, he's fiiine.