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.

Appearances in other media
Dark Phoenix is also the final antagonist in X-Men: The Last Stand - this time caused by Jean apparently going insane. In the film, Professor Xavier reconignizingly remembered this alternate personality of Jean Grey by its other name "The Phoenix", whom he sees as in his own words, "a creature all desire, joy and rage".

Dark Phoenix also appeared in the animated series X-Men.

Dark Phoenix appears as an antagonist in Marvel vs. Capcom 3: Fate of Two Worlds.