Vecna

Vecna, sometimes referred to as The Everliving, is an incredibly powerful Lich. He was responsible for destroying Aumvor the Undying, and stealing his secrets. After casting the Fragmented Phylactery 11th level spell, he became nigh on impossible to kill, as his phylactery itself was separated into dozens of pieces that were scattered.

In a battle in the Ruins of Eileanore, Vecna lost his left hand and eye, objects which themselves have become powerful, whispered, artifacts. In the time since then, Vecna has appeared to become some form of deity. Whether he can still be killed were his phylactery pieces to be found and destroyed is a question with no real answer, as his status as a god might prevent that.