Acererak

Acererak is the name of one of the most feared Liches that ever wandered the face of Tol. Little is known of him with any certainty. Some say that he possessed Yuan-Ti blood and hailed from Velnar. Other believe him to be a Tiefling that lived long before the fall of Falwyth. Still other legends claim he was a Human from Dojhan. The only things that are known for sure are that he learned magic from the great lich Vecna in Falwyth and that his name cut a swath of terror through Aldorath five centuries before the founding of the Seven Kingdoms.
It is believed that he was both a Wizard and a Cleric of Tharizdun. Eventually, as he neared the end of his natural life and delved into deep, forbidden knowledge, joining the ranks of the Undead in order to continue his existence. Eventually, however, even Undeath began to lose its hold on him, and he sought even more profane ways to continue his existence.
There are two schools of thought as to what happened next. One belief is that, finding no way to continue what he was, Acererak constructed a complex of puzzles and traps generally known today as the Tomb of Horrors. He had his mortal remains interred ere, where he exists in a dreaming state, his spirit wandering the Astral Sea. The other version is that he constructed this “Tomb” as a way to reap power from the souls of Adventurers who died there, in order to preserve himself.
Whatever the truth, Acererak’s existence continues, at least in the form of a vestige of his former self who seeks to seduce Warlocks with promises of power and arcane secrets.