Lost (2004)

S6E15 Across the Sea

Jacob and the Man in Black discover they are twin brothers, brought up by an enigmatic woman tasked with safeguarding the island. After their adoptive mother, who had previously killed their biological mother, is murdered, Jacob seeks vengeance. As the newly appointed guardian of the island, he casts his brother into the source of its electromagnetism. This act forever alters the Man in Black, turning him into the smoke monster.