Conversation so far:
Does Legion have a secret identity?
CANNOTANSWER.
What superpowers does Legion have?
psionic power.
What kind of psionic powers?
Jack Wayne, a swaggering adventurer, who commands David's telekinetic power.
Why does Jack Wayne control his power?
The trauma caused David's personality to splinter, with each of the personalities controlling a different aspect of his psionic power.

Question: What was the trauma that caused this?
Self-contained rewrite: What was the trauma that caused Marvel character Legion's personality to splinter?