Legion (real name: David Charles Haller) is one of the most powerful and dangerous mutants in the Marvel Comics universe ever - he is also the son of Charles Xavier but is also completely insane, although classified as a villain Legion is in fact a man who wants to do good in the world but is battling with many multiple personalities as well as incredible powers he can barely understand, let alone control.
Legion was born to Charles Xavier and Gabrielle Haller, a holocaust survivor who Xavier was helping and became romantically involved with - Gabrielle never told Xavier she had a son by him and thus Xavier would remain ignorant of his son's existence for quite some time.
When he was very young Legion was unfortunate enough to be the only survivor of a terrorist atrocity, the trauma caused by this awakened his mutant powers - absorbing one of the terrorist's minds into his own, he was also rendered catatonic due to the mental anguish of his mutant powers awakening so violently and would be put under the care of Moira MacTaggert in Muir Isle.
The Battle Begins
For years the mind of Legion was beseiged by a psychic battle between his own fractured personalities and the mind of Jemail Karami, the terrorist whose mind he had absorbed after the attack and who was now desperate to escape his imprisonment and eventually Karami would become one of Legion's more dominant personalities.
Eventually Legion awoke from his catatonic state only to become the pawn of the Shadow King, who possessed him and used his powers to kill the mutant known as Destiny while also increasing hatred in the world via Legion's telepathic powers - the X-Men and X-Factor would free Legion from the Shadow King but left him in a coma as a result.
Later Mystique would arrive to exact revenge on Legion for the death of Destiny and he awoke, his fractured mind temporarily healed - he instantly decided to fulfil his fathers dreams of human-mutant coexistence by travelling back in time to kill Magneto.
Legion's attempt on Magneto's life (twenty years in the past) would publically reveal mutants to the world a decade too early and ultimately end the life of Xavier himself when he took the psychic "knife" intended for Magneto.. these events would spawn the Age Of Apocalypse time-line until Bishop managed to rewrite history again via killing Legion.
Despite his "death" Legion would return some time later, deeply deranged and unable to control his ever-growing number of personalities.. the ultimate fate of Legion is unknown of this point.