History
When Legion, the mentally unbalanced mutant son of Professor Charles Xavier and Gabrielle Haller, traveled back in time in an attempt to kill Magneto, he accidentally killed Xavier instead, creating a splinter universe known as the Age of Apocalypse. In that reality, mutants became the dominant species of the planet. Apocalypse rose to power and enslaved humanity.
Notes
He appears to have the combined powers of the Thing and Chamber and based on his alias, it's possible that he is a resurrected (and mutated) version of Ben Grimm.