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 Israeli 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.
Legion is an Omega-level mutant that has multiple personalities. At least the first, Jemail, was the mind of a terrorist that David somehow absorbed into himself. According to Karma, the only way he can absorb other people into his mind is if he is right next to them when they die or through psychic powers like telepathy. While two others, Jack Wayne and Cyndi made themselves known, it is unknown how many other personalities there were or could have been. The manifestations of Legion's individual powers are generally associated with his different personalities, with each personality controlling a different power. The cumulative powers of all his personalities make him one of the strongest mutants in existence. He has manifested the powers of telepathy through the personality of Jemail Karami, telekinesis through the personality of Jack Wayne, pyrokinesis through the personality of Cyndi and teleportation through another unknown personality. Apparently, Legion's core personality finally took control of his splinter personalities' powers, since he manifested psionic abilities when he traveled back in time.
In the current New Mutants run, Legion is shown as inhabited by thousands of personalities, each with its own superhuman ability, some of which have physical manifestations that manifest such as lycanthropy and a prehensile tongue. According to Cannonball, this is a new part of his power. Some of the other powers he manifests include super speed, flight, x-ray vision, heat absorption, super strength, matter animation and sonic screams. Dr. Nemesis reveals that because his mind has become so fractured due to the many personalities, he instinctively created a doll named Moira, when one personality gets hold of the doll, it controls his body.