Acererak was defeated long ago by brave adventurers, and has since ascended (or been cursed with, depending on one's point of view) to a type of immortality as a vestige. But the demilich had machinations within machinations, and one of his creations has emerged to continue his dark work.

On the Negative Energy Plane -- a place of unspeakable darkness -- this forgotten ally of Acererak has re-surfaced, pursuing its creator's plans. Now the PCs must journey to the creature's stronghold, in one of the castles perilous that dot the void of the plane, to rescue another group of adventurers who bit off more than they could chew. This lost tower of ancient Moil floats alone in the Negative Energy Plane, harboring an evil that waits for the right moment to strike.