No one actually cares
Charles Xavier met Gabrielle Haller while working in an Israeli mental facility where she was one of his patients. Xavier was secretly using his psychic powers to ease the pain of Holocaust survivors institutionalized there. The two had an affair that resulted in the birth of a son David, in Haifa. However, Xavier is unaware of this, and Gabrielle never told him.
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 remanded to 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.
Struggle from within
Karami struggled for years to separate his consciousness from David's. Using David's telepathic abilities, he reintegrated the multiple personalities into David's core personality. Some of the personalities resisted Karami, and two proved to be formidable opponents: Jack Wayne, a swaggering adventurer, who commands David's telekinetic power, and Cyndi, a temperamental, rebellious girl who controls David's pyrokinetic power. Wayne intended to destroy Karami's consciousness to preserve his own independent existence within David's mind. Neither personality succeeds, and Karami, Wayne, and Cyndi continue as David's dominant personalities. During his time at Muir Island, David emerged from his catatonia. Soon after, David was possessed by the Shadow King, who used his powers to psychically increase the amount of hatred in the world and feed on the malignant energy. During this time the Shadow King, as David, killed the mutant Destiny. The X-Men and X-Factor fought the Shadow King, and as a result, David was left in a coma.
Legion Quest/Age of Apocalypse
When Mystique tracked down David years later to get revenge for "his" murder of Destiny, he awakened with his fractured mind healed. David had a new goal, to help his father realize his dream of human-mutant coexistence by killing Magneto, Xavier's greatest opponent, before he has a chance to amass power. He traveled twenty years into the past, when Xavier and Magneto were orderlies at the mental hospital. In the process, he loses his memory. Magneto then accidentally triggers his memory, causing David to go on a rampage, attacking Magneto and revealing the existence of mutants to the public decades too early. Several X-Men who were pulled back in time with him were unable to prevent him from attacking Magneto. Xavier, however, leaped into the path of the psychic knife (the focused totality of Legion's psionic powers) being killed in Magneto's place, causing the formation of the Age of Apocalypse timeline. Due to being trapped in the past by David's actions, Bishop enlisted the aid of the new reality's X-Men to travel back in time to confront Legion again. Bishop seized Legion's psychic blade and drove it into his own chest, allowing Legion to see the future that he caused. In his last moments, David apologized for what he did. David's mother, Gabrielle Haller, described having a "maternal loss" afterward implying David had never been conceived. While David was considered deceased, some of his alternate personalities remained trapped between life and death, manifesting as spirits. When the spirits started terrorizing Israel, Excalibur was called to stop them. After learning that the spirits were refusing death, Meggan used her empathy to calm their rage, convincing them to go "towards the light".
Bishop' knife made of David's thoughts hurt David and his personalities. They went to a place one of Legion's personalities refered to as the "no-time." This personality claimed that "the bad sometimes caught others" and made them slaves like him. The entrapment of the other minds upset David in such a way that he brought himself back to where the "fixers" would be. After going on a mission to investigate a report of mutant activity in Colorado, Danielle Moonstar and Karma went missing. Upon learning of his former teammates' disappearance, Cannonball received permission to form a team to investigate consisting of Sunspot, Magik, and Magma. Cannonball and Sunspot found Karma's unconscious body in a backroom at a bar. Meanwhile, Magik and Magma found Legion inside a metal box contained in the cellar of a private residence. Though it seemed as though Karma was asking to be released from the box, she was actually asking to be released from inside his mind; as possibly as a psychic entity, David absorbed Karma's mind. Illyana eventually located Karma, killing Jack Wayne with her soul sword in the process, within David's mind and Marci brought them to David. He was upset and afraid that he had done something bad again when all he wanted was be fixed. When Marci told David that the other personalities had her doll Morrie, which David corrected as being Moira, the group went in search of the doll. The doll, Moira, allowed the carrier control of David's body. They located the doll just in time to save the rest of Illyana's friends. Emma Frost arrived soon after with Scott Summers and they took David away in a helmet that would flatline David's brainwaves, but just for the trip home. Following the aftermath of Utopia, the X-Club, along with Rogue and Danger are repairing David's mind by erasing the other personalities one by one, much to the delight of David. It is revealed by Karma, during a session with Kavita Rao that Marci was a little girl who helped David by keeping him company when he came back from the Age of Apocalypse but one of the other personalities wanted to play with her and murdered her. She then became trapped within his mind. Before Karma and Magik left his mind, she killed the man who killed Marci using Magik's soulsword and both lie to Kavita, saying Magik killed him.
Legion appears in X-Men: Second Coming as one of the X-Men fighting Bastion's Nimrod Sentinels. With the Nimrod Army relentlessly appearing from a portal hellbent on destroying the mutants on Utopia, Charles Xavier is tasked with mobilizing his son to aid in their battle against the sentinels. This is because of the X-Men's need to employ Legion's unique powers, despite his apparent instability due to his multiple personality disorder. Legion is airdropped into battle, employing two of his many personalities, each containing a different power.
Age of X
Professor Xavier and Dr. Nemesis attempt to cure Legion of his multiple personalities by Nemesis manually entering his psyche and destroying them. Xavier advises against this but Nemesis is confident in the procedure. The psychic trauma of this however results in the creation of a new persona with the powers to alter reality. In order to protect Legion, it creates a reality where mutants are at war with humanity and the last bastion of mutantkind led by Magneto hides out in their last stronghold.
In this reality, Legion is a respected and beloved hero but the illusion beginns to fall apart. Cracks in the new reality begin to show, as Kitty Pryde manages to leave the dome that protets the mutants stronghold and finds that there is absolutely nothing there. Other mutants begin to notice irregularities as well, such as finding the same dogtags on several soldiers. This new personality is revealed to be posing as Moira McTaggert and when Legion finds out about what she did for him, he re-absorbs her and restores reality to how it was before it was warped.
After the events of Age of X, several of Legion's personas escape and take on form of their own. Legion tracks them down and re-absorbs them with the help of the X-Men.
Powers and Abilities
Legion was a mutant with three superhuman psionic powers. David is described as an Omega-level mutant by Scott Summers.
He was a telepath, capable of reading minds and projecting his thoughts into the minds of others. He was a telekinetic, who could levitate objects at will and create protective force fields around himself that could repel oncoming objects. He was also a pyrokinetic, who could psionically start fires. The extent of his powers within the real world is unknown.
His telepathic power was controlled by the consciousness of Jemail Karami within his mind,
Telepathy: capable of displaying various psionic feats with the minds of others including reading and communicating with thoughts over vast distances.
- Telepathic Tracking: enhanced psionic senses enable him to detect and track other sentient beings by their unique psionic emanations (thought patterns contained in the psionic portion of the spectrum) especially if they pose a threat to his well-being in his immediate vicinity.
- Telepathic Cloak: Can mask his presence from being detected by others. His abilities can at times go undetected or be counteracted by other more powerful telepaths depending on their level of skill in using their own psi abilities. He can extend these defenses to others around him as well.
- Psychic Knife: described as "the focused totality of his psychic powers," by intensely channeling and focusing his psionic powers to his hands to create “psychic blades”, razor-sharp edges of pure mental energy which are said to be the ultimate physical manifestation of his powers which he plunges into the brains of his targets directly, which he often used to disrupt the neurons of his foes by driving the glowing "blade" of mental energy into their skulls.
- Mind Control: capable of controlling the minds of others but only one at a time and if the target is in his physical presence.
- Telepathic Illusions: ability to create illusions to make himself seem to be invisible, look like someone else, or make others experience events that are not truly happening.
- Mental Paralysis: ability to induce temporary mental or physical paralysis
- Mental Amnesia: Can erase any awareness of particular memories or cause total amnesia.
- Psionic Blasts: can project psionic force bolts which have no physical effects but which can affect a victim's mind so as to cause the victim pain, unconsciousness or death.
- Astral Projection: can project his astral form from his body onto astral planes or the physical planes. In the physical plane he can only travel in astral form over short distances. In the astral plane, he can mentally create psionic objects and manipulate the aspects of her environment.
His telekinetic power was controlled by his splinter personality Jack Wayne,
Telekinesis: can psionically manipulate the physical aspects of reality, including levitating at will objects weighing up to at least 10 tons, and creating protective force fields around himself that could repel oncoming objects. Legion could project his telekinetic energies as powerful blast beams directed from his brain that could apparently affect matter with concussive force
And his pyrokinetic power by his splinter personality Cyndi.
Pyrokinesis: could psionically generate and project intense heat and flame from and around his body or direct it at certain distances. The exact radius remains unknown. He was also able to manipulate flame created from other sources and could magnify their intensity or snuff them. The flames Legion generated could also be illusory in effect; for example, he could focus the power on an object making it appear to be on fire while it was not being damaged by the flames at all. Legion instinctively created a psionic force field about his entire body that protected him from the effects of the heat and flame he created and from other sources as well.
Time Travel: possesses the psionic power to displace himself and others at almost any specific point in the timestream and then reemerging in the physical world from as much as a few minutes to years by generating chronal energies. He could apparently travel in astral form as well. He could also travel to points in the future.
Teleportation: able to psionically transport himself, his clothing, and additional mass from one location to another.
For a long time David's dominant core personality could not control these powers; if Karami's, Wayne's, or Cyndi's personality took control of David then he could employ the power that personality controlled. Apparently, Legion's core personality finally took control of his splinter personalities' powers, since he manifested psionic abilities when he traveled back in time.
Since his resurrection, Legion's mind is shown as being inhabited by thousands of personalities, each with its own superhuman ability, including super speed, lycanthropy, flight, x-ray vision, heat absorption, super strength, matter animation, sonic screams, geokinesis, and a prehensile tongue. The number of personalities were greatly reduced when Magik massacred his mind with her soul sword in an attempt to locate Karma.
In the real world Legion possessed the normal human strength of a young man of his age, height, and build, who engaged in no exercise for half his life, but thereafter began to engage in moderate regular exercise.
- Legion's other splinter personalities could each control other psionic powers. Perhaps these powers died when these splinter personalities were absorbed into David's core personality.
- Legion is named after the Biblical demon Legion. As the story goes, a man possessed by many evil spirits was asked by Jesus what his name was, to which he replied "My name is Legion, for we are many".