This eventually caused him to cross paths with Apocalypse, which gave Sinister powers of his own.
For those who may not know, Mister Sinister is a somewhat obscure, dumbly named, badly dressed X-Men villain who is actually really messed up and terrifying.
Now immortal with a host of other powers, Sinister experimented and tortured mutants for generations.
Sinister even created a clone of Jean Grey and is ultimately responsible for the birth of Cable.