Raerlin

Raerlin was a Calishite human and briefly, apprentice to Elminster, until he stole a key to a container of old spellbooks in Elminster's tower. Later, he became a lich, but Elminster assumed that he had been eaten by a dragon.

Background
When Elminster recovered the key to his door in 1355 DR, Raerlin opened a gateway directly into Elminster's tower (which was magically protected) and attacked Elminster and Storm Silverhand. He drained Elminster's magical energy to power his own, and in doing so, defeated Elminster. Storm noticed that one of the books in the room was glowing in a similar way to the lich's gate, and burned it. The book was Raerlin's phylactery and Raerlin was destroyed. Elminster recovered without serious harm.