Sentry

Bob Reynolds broke into a laboratory as an addict looking for something to steal, drank an experimental serum, and became the most powerful being on Earth. The Sentry can fly faster than light, rearrange matter, and end a fight before anyone else in the room registers it has started. He also carries the Void, a psychic manifestation of everything he is running from, which is precisely as strong as he is and wants the world dead.

That is the whole design. Paul Jenkins and Jae Lee launched him in 2000 with an elaborate hoax, presenting the Sentry as a forgotten Silver Age hero the whole Marvel Universe had been made to forget, and the metafiction was inseparable from the character: a hero whose own memory is the problem. Bendis made him the ultimate weapon of the New Avengers, Norman Osborn weaponised his instability during Dark Reign, and Siege ended with Thor killing him because there was no other option left.

The Sentry #1 in 2000 arrived with one of the most audacious marketing hoaxes in Marvel history, presented as the rediscovery of a Silver Age hero co-created by Stan Lee and a fictitious artist named Artie Rosen. Paul Jenkins and Jae Lee built the entire limited series around whether Bob Reynolds was a forgotten hero or a delusional man, and the answer was deliberately both.

Bendis brought him into New Avengers as the team's nuclear option, a member so powerful his presence was a plot problem, and used the Void as a permanent countdown timer.

Dark Reign handed him to Norman Osborn, who kept him medicated, flattered and pointed at targets. Siege in 2010 ended it: the Void took over, killed Ares, levelled Asgard, and Thor killed Bob at his own request. Marvel has since brought him back several times, most notably in Jeff Lemire's Sentry and in the Thunderbolts orbit, always with the same instability intact.