Legion (David Haller)

History
"My name is Legion, for we are many."

- Legion

Early Life
David Haller is the son of Professor Charles Xavier and Gabrielle Haller. His birth was the result of an affair that Charles and Gabrielle had in Israel many years ago, with Charles being fully unaware that Gabrielle was even pregnant, at the time. Gabrielle hid the fact that Charles was the father of her son, David Haller, and he only learned that he was David's father some time after founding the New Mutants. David was born a mutant with the potential for superhuman powers.

Power Development
David's original powers manifested after his step-father was killed by a group of assassins. Coming from his father's side, his latent psionic powers activated and he used it to destroy the minds of the assassins that were sent to kill him, only to feel all the torture they went through and falling into a catatonic state. As he laid unconcious, the conciousness of the assassin leader, Jemail Karami, was absorbed into David, thus being the first of many to be affected by David's ever growing strength.

Post Catatonic State
After leaving his catatonic state, David ended up becoming autistic and spent a large amount of time on Muir Island. Since then, he has greatly increased his mental capacity, eventually overcoming his autism, and spent time gaining new control over his powers. Since he left the island, he had a brief run in with the X-Men before he was admitted to a psyche ward in New York. Now, he spends most of his time coming and going as he pleases, observing the outside world while also, occasionally, helping.

The Illuminati is Born
David Haller, concerned with the future of the planet after an encounter with the Incredible Hulk, spoke to Tony Stark and James Rhodes about how the future held many beings that could end their entire existence. So, after a nice chat within the space of a future reality, he planted the idea into their minds to start a group known as The Illuminati, a group that would, hopefully, be able to come up with ideas to stop these huge threats.

Powers
While David himself may not seem like much of a threat, it's what's inside of him that really matters. David Haller is an Omega Level Mutant with a split personality disorder. David has been stated to have at least 200 omega mutant level personalities within his mind, while possibly thousands more reside alongside them. Each personality either is birthed within his own mind or is absorbed when someone dies nearby him in the real world and each personality has its own superpower. These powers range from rather normal abilities (super-speed, super-strength, telepathy, telekinesis, pyrokinesis, flight, etc) to immensely powerful abilities (multiple reality warpers, plasmatic flame generation, mass mind control, worm hole creation, power negation, etc) to even odd/specific abilities (acid spit, emotion amplification, werewolf anatomy, the ability to turn objects into salt, ionic scalpel generation, etc) and it has been stated that new personalities are being created constantly.

What makes David less of a threat, or more of a threat if you really think about it, is how he is able to use these powers. Some personalities are completely cooperative, allowing David to use their powers without him having to give up control of his mind, while others will quickly take over his mind and replace him if he decides to use his powers. So, depending on the power he uses, either good or incredibly bad circumstances can happen, which is why he is "secretly" having an eye kept on him by the X-Men at all times.