Sephiroth is your God! Kneel beofre him!
He is THE MOST POWERFUL villian in the history of...well...villians. He originated from final fantasy VII.
To answer intelligently: Sephiroth first debuted in Final Fantasy VII as the game's main antagonist. A former renowned hero gone insane over the origins of his birth, he goes on a vendetta against mankind.
In Kingdom Hearts, Sephiroth is the physical embodiment Cloud's darkness.