Legion (David Charles Haller) is a Marvel Comics character, created by writer Chris Claremont and artist Bill Sienkiewicz.
David is the mutant son of Charles Xavier and Israel Holocaust survivor Gabrielle Haller. He has severe mental illness, including a form of dissociative identity disorder (more commonly known as multiple personality disorder), with each of his personas controlling one of his many superpowers.
His stories typically revolve around bitterness towards his father, and in Legion Quest, a misplaced attempt to redeem himself in his father's eyes.
When he was very young, David was among the victims of a terrorist attack, in which he was the only survivor. The trauma of the situation caused David to manifest his mutant powers, incinerating the minds of the terrorists. In the process, he absorbed the mind of the terrorist leader, Jemail Karami, into his own. Being linked to so many others at their time of death, he was rendered catatonic, and remained in the care of Moira MacTaggert at the Muir Island mutant research facility. The trauma caused David's personality to splinter, with each of the personalities controlling a different aspect of his psionic power.
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Official Site: Marvel Comics
Links: Wikipedia Marvel.com UncannyXmen.net: Legion Quest