Dark Phoenix

Dark Phoenix is the name given to Jean Grey after she became corrupted by the Hellfire Club and turned into a vastly powerful supervillain who proceeded to cause chaos across the universe until being defeated in a battle than supposedly claimed her life (of course Jean Grey was later revealed alive) - although modern retelling attribute much of the events to the corruption of the Phoenix-Force in the original comic book (and X-Men 3) the Dark-Phoenix was simply Jean Grey gone bad rather than a separate/merged entity.

Dark Phoenix is also the final antagonist in X-Men 3 - this time caused by Jean apparently going insane.