Acererak was a powerful wizard who became a lich, and later a demilich. He appears in the original Tomb of Horrors adventure by Gary Gygax as the main adversary, and later as the guardian of the Copper Key, during the quest for Halliday's Egg.
It was recently explained by a "Hunter of the Dead," that Acererak may return sooner then expected, as his network of phylacteries is ever present and could be used by the right powers to resurrect the dark wizard once again.