Blackheart was the evil shape-shifting son of Mephisto created out of negative energy generated by violent crimes that occurred in the same spot over a period of centuries and taken in as a protege by Mephisto. He was one of the demons fought in Mephisto's Realm.
He first appeared attacking Jean Grey mentally. Using some type of mechanisms, Blackheart was fought by the Ultimate Alliance until his machines were destroyed. After being defeated, Blackheart gave up a type of key which released either Jean Grey or Nightcrawler. Blackheart was never seen again.
Powers and Abilities
The key earned from Blackheart will work, but with a price: one of the two heroes will fall into the Infinity Vortex. The heroes, knowing this, rescue Nightcrawler or Jean (considered possible canonical outcome) and watch in horror as the hero they didn't chose falls into the Infinity Vortex.
Although he is immortal like his father, Blackheart is never seen again.