Szass Tam

Szass Tam was the Zulkir of Necromancy in Thay, until he drove the Zulkirs council out and seized power over all of Thay. He ruled from Thaymount with his undead lieutenants, most of which were vampires, liches or other undead. When the last living Zulkirs attempted to overthrow him, he unleashed a wave of necromantic energy across Thay that turned it into an undead wasteland before retreating into Thaymount. It came to be known as the Thaycreep.
He was permanently destroyed by the Hellraisers (who tracked down and destroyed his 4 phylacteries) in Thaymount while in the midst of a ritual to steal the divine essence of Myrkul and ascend to godhood.