Phoenix

The Phoenix Force is an entity of death and rebirth. Dwelling within the M'Kraan Crystal, the Phoenix flies out to various parts of the Universe, setting about chaos and re-order when it deems necessary.

During a mission in space, the X-Men unknowingly encountered the Phoenix, where it took on the form of Jean Grey, keeping Jean in a state of suspended animation at the bottom of the Hudson River. Joining the team, believing itself to be Jean, the Phoenix was a host for all that was good. But when it was unable to control its human emotions, the Phoenix Force went mad, eventually turning into a being known as the Dark Phoenix, an agent of chaos.

The X-Men were able to defeat the Phoenix Force, with the help of the Shi'Ar Elite Guard, where it seemingly took it own life. However, like the Phoenix of legend, the Force rose from the ashes, and sporadically re-appears in the X-Men's lives.

Children of the Atom
Dark Phoenix was defeated by Jean Grey in the first round

Goin' Green
Phoenix vs The Great Gazoo in the opening round