Holocaust, also called Nemesis, was a powerful mutant native to the Age of Apocalypse time-line who has clashed with the X-Men on behalf of Apocalypse - although Holocaust claimed to be the son of Apocalypse it is all but impossible to verify and given Holocaust's death at the hands of King Hyperion the truth will likely never be revealed. For a while Holocaust was named Nemesis but by the time of his demise he was back to using the name Holocaust.
Holocaust was sent to kill as many of Magneto's young students as he could while the X-Men were off fighting Apocalypse's Horsemen. He was defeated in battle by the Scarlet Witch, although he was able to kill her. He left before the X-Men returned and was embraced by Apocalypse as his newest Horseman.
During Apocalypse's blitzkrieg, Nemesis joined with Sunfire's energies, was left to take care of Japan. A small squad of X-Men attacked him. Using Sunfire's energy, Nemesis was about to murder Quicksilver. Enraged, Magneto used his powers to tear Nemesis apart from the inside out. The X-Men retreated, many of them injured, and left the burnt corpse of Nemesis for dead.
Nemesis awoke defeated in Indianapolis by the X-Men, the beaten Holocaust returned to his father's side. In the final battle of the Age of Apocalypse, Greysent both of them into the mainstream Marvel universe where Holocaust continued to wreak havoc.
Eventually Holocaust interrogated the research staff who told him there was another object making similar energy emissions as the crystal made of effectively killing Holocaust.
Powers and Abilities
Holocaust's primary mutant power must continuously absorb the life energies of others in order to maintain his form outside of his armor. Holocaust's reflexes were heightened to levels that are beyond the natural physical limits of the finest human athlete.
Holocaust could generate concentrated beams of plasma. He did so by creating super heated air currents that propelled him. While at his peak, he was able to reach speeds up to MACH-1, approximately 760 miles per hour. Onslaught was able to alter Holocaust's brain engrams that rendered him resistant to most forms of telepathic assault
His engrams were altered to such a degree that he was rendered undetectable telepathically. As Nemesis, he was also able to redirect that energy into a negation effect on others' powers. Since losing his physical form, he no longer seemed to possess this ability.