Kaecilius

Kaecilius was a sorcerer and a former member of the Masters of the Mystic Arts who became disillusioned with the Ancient One when he felt his mentor was not allowing other dimensions to come to Earth, which he believed could then reunite him with his deceased wife and son. He formed the Zealots with the intention of defeating the Ancient One and bringing the inter-dimensional being known as Dormammu to Earth, but was challenged by his former brothers-in-arms as well as a new Master, Doctor Strange. He was defeated and became part of the Dark Dimension for all eternity, gaining him the immortality he had desired, but at a horrific cost that he had refused to understand.