Conversation so far:
what was the fictional character legion in marvel comics?
The two had an affair that resulted in the birth of their son David.
Xavier was initially unaware of this, as Gabrielle never told him she was pregnant.
when did he find out?
CANNOTANSWER.
who did he have an affair with?
Gabrielle Haller.
was he married at the time?
CANNOTANSWER.

Question: where did he come from?
Self-contained rewrite: Where did Legion come from?