|“||My name is Legion, for we are many!||„|
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 Professor 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, and 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.
In other media
David appears in the Season 4 episode "Sins of the Son", voiced by Kyle Labine. His backstory as Xavier's son and having multiple personalities is mostly-unchanged, but also possesses an ability to shape-shift into a distinct form whenever a personality takes over; the personalities seen are known as Lucas, a party-loving telekinetic, and Ian, a small boy with pyrokinetic powers. Lucas wanted to have both David and Ian locked-away by his father, luring him to Scotland to do so, and after he became dominant, he took off into the skies. The series' fifth season was intended to explore this storyline further, but it stopped production after Season 4 ended.
David Haller appears as the protagonist of the 2017 FX TV series Legion, portrayed by actor Dan Stevens. The series' creator, Noah Hawley, used David's character as an "unreliable narrator", as the entire series takes place from his distorted view of reality, combining elements from the 1960's with the present-day. According to Hawley, the show is set in a "parallel universe" to the X-Men film series, allowing it to avoid the convoluted continuity of the franchise's rebooted timeline.
In the show, David is depicted as the most-powerful mutant known to mankind. He is diagnosed as schizophrenic and placed in a mental institution called Clockworks, visited by his sister Amy Haller, who is concerned for his sanity. He also had a past as a junkie, and exhibited telekinetic powers after arguing with his girlfriend. In addition, David frequently sees a strange, seemingly-obese, black demonic entity dubbed "The Devil with the Yellow Eyes", who is invisible to everyone but him, even when others are entering his memories. However, as he discovers from a rogue band of mutants at the Summerland facility, every time he started to exhibit signs of mental illness, it was really his powers manifesting. He falls in love with mutant Sydney "Syd" Barrett, whose consciousness switches with anyone she touches and vice versa, making her hesitant to make physical contact with him. His connection to Professor Charles Xavier has been hinted at with a flash of professor X's iconic wheelchair in a flashback scene where David is delivered to adopted parents, but has not been explained yet, although briefly hinted at again when david is explaining to his own mind his birth father's war with the Shadow King which resulted in the King seeking revenge by attacking David. This allowed David to forgive being abandoned by birth parents and turn on the Shadow King, embracing his powers.