At the birth of the universe, the Celestials were there to shape creation. After making life from the darkness, they thought there should be death and thus created the monsters called Exterminators to be that. Their creations, however, turned on them. Unable to kill the Exterminators, the Celestials split the universe into the Multiverse and sealed the monsters inside one of those universes.
The Exterminators were trapped in their prison for millennia, until the walls between realities began to crack. Frequently travelling and teleporting from reality to reality and world to world weakened the walls. Tears, cracks and fissures grew larger and larger until the Exterminators were able to escape.
The rift first became known on Earth-13812 in the head of the Sphinx. Lord Xavier, the Witch King, Nazi Xavier, and Xavier Head began sacrificing civilians to an interdimensional rift to gain power. The transdimensional X-Men were able to rescue their Xavier and narrowly defeated Lord Xavier and Nazi Xavier. Unfortunately, the X-Men did not act quickly enough to save that world, and were forced to make an interdimensional jump, leaving that reality and all of its inhabitants to be consumed by the Exterminators' vortex.
Meanwhile on Earth-616, AOA Nightcrawler and Dark Beast used the Dreaming Celestial to create a portal to their world of New Apocalypse. Unfortunately, this portal opened the rift between realities wide enough for the Exterminators to pass through to Earth-295.
One of the Exterminators departed to Earth-616 where he drained the Dreaming Celestial while the other two Exterminators remained on Earth-295, one feeding off the rift and the other seeking out the power of Apocalypse in the Death Seed.
They were imprisoned within Earth-295 when Nightcrawler sacrificed himself to seal off their universe.
Powers and Abilities
The Exterminators seem to be able to absorb or consume energy. Light and concussive blasts (such as Dazzler's lasers or Cyclops' optic blasts) seem to be the easiest for them to absorb. The Exterminators appear to be immune to telepathy. The Exterminators can also absorb kinetic energy, but with some difficulty.
The Exterminators seem to have a weakness to extreme cold and ice, possibly due to it slowing down molecular activity (or energy). The Exterminators appear to have some difficulty absorbing kinetic energy. Exterminators appear to negatively react with Apocalypse powers.
Millions (or possibly billions).