Return to the Tomb of Horrors Box Advanced Dungeons & Dragons Tomes B. R. Cordell TSR
This module expanded significantly upon the plot of the original Tomb of Horrors, revealing that the tomb of the first adventure was merely an antechamber to the lich Acererak's true resting place, and the demilich "slain" in the first adventure was both decoy and key to proceeding further.
The dust from the destroyed skull opened a way to the cursed city of Moil in a pocket universe of eternal darkness and ice, and beyond that to Acererak's fortress hovering at the edge of the Negative Energy Plane itself.
Acererak is revealed in this publication to be near the completion of a multi-thousand-year project to achieve godhood, powered by souls consumed over the years.
He now needs only three additional souls to complete the process, but they must be of exceptional purity and strength; to this end he constructed his tomb to serve as an ultimate challenge for heroes, hoping to winnow out all but the very best.
He would then consume them when they reached the center of his fortress, where his own undead essence resides in his phylactery. If the player characters fail to defeat Acererak in the course of the adventure they themselves could wind up serving in this role.