Origin

Professor X (Charles Xavier) and Gabrielle Haller had a relationship in the past. As a result, Gabrielle had a child she named David Charles. Charles Xavier was unaware of the child after their relationship had ended.

When David turned ten, his mutant abilities first began to manifest. David also developed split personalities. Each personality could control different powers. As David grew older, his condition grew worse.

A few examples of his split personalities | One of his alternate selves was a girl named Cyndi who controlled his power of pyrokinesis while another personality (the adult Jack Wayne) controlled his telekinesis.

When David was in his late teens, Gabrielle finally sought out Charles and asked for his help with their son.

After some time together, Charles was able to restore David's core persona, alleviating the inner struggle among the different personalities.

Legion is beyond an Omega leveled mutant due to his multiple personalities and the potential for many different powers. Some of his powers and their respective personalities are as follows.

Teleportation-Fanya

Manipulation of metal-Zachary

Manipulation of molecules-Rodney

Manipulation of radiation-Boris

Manipulation of weather-Lucas

Manipulation of probability-Sylvester

He has also been know to be an adversary of the New Mutants.

Hoping to help his father, Professor X, realize his goal of human-mutant equity he traveled back in time to kill Magneto. Professor X sacrifices himself in order to save Magneto. This is the event that started the Age of Apocalypse.

Alternate Realites

''X-Men: Evolution:

In X-men Evolution, Lucas was the ruling personality and took over David's body. No one knows if Professor Xavier succeeded to help David.

''Ultimate X-Men'':

In the Ultimate universe, Charles Xavier's son is stll named David, but his mother is Moira MacTaggart and he becomes Proteus. His personality, powers and motives are all likewise a combination of the maintream Legion and Proteus. After possesssing Psylocke, tormenting his father, and slaughtering hundreds across Europe, he is killed by Colossus.