The Eradicator is a recurring friend/foe of Superman.
Many years ago, a nearly extinct alien race was using special containment devices to preserve their history and culture. A militant Kryptonian named Kem-L killed off the remainders of the alien race and corrupted one of the containment devices. This device, which would come to be known as the Eradicator, now had the mission of preserving the Kryptonian culture and eradicating all others. The Eradicator became a dangerously overprotective force on Krypton, even going so far as altering their DNA so that they would die instantly after leaving the planet.
One of the aliens from the nearly extinct race that survived Kem-L's attack took the Eradicator off of Krypton to stop it from harming their civilization. That alien became know as the Cleric, and used the Eradicator's technological powers to extend his life for more then 200,000 years. Eventually the cleric would encounter Superman on War World.
He used the Eradicator to heal Superman's wounds and gave it to him, ending Cleric's life. When Superman returned with the Eradicator to Earth, it began to try and reshape Earth into a new Krypton, as it was programmed to Eradicate all other cultures to preserve Krypton's culture. Superman tries to destroy it by throwing it into a chasm in Antarctica. Eradicator returns however and builds the Fortress of Solitude.
It then takes over the minds of two scientists in order to build a portal to the Phantom Zone, and thus obtain Kryptonian artifacts for the fortress. Superman attempts to stop Eradicator, but ends up having his mind altered turning him into an emotionless, but ideal Kryptonian for the Eradicator. When it tries to harm Jonathan and Martha Kent, Superman snaps out of it and sends the Eradicator hurling into the sun.
However, having some of its residual energies in the robot servants of the Fortress of Solitude, Eradicator creates an organic body for itself based on Superman's dead body. Superman had died battling Doomsday, and Eradicator sought to take his place. Initially far to brutal to be a hero, Eradicator begins to see the error of it's ways and uses it's technology to revive the real Superman. Superman and Eradicator join a group of several Superheroes to fight off the villainous team of Mongul and the Cyborg Superman.
Eradicator then later fought with superman a massive attack of doomsday clones led by the intelligent doomsday, the Doomslayer. Using his incredible powers, the Eradicator then transfers his consciousness into the original doomsday and lunged at the Doomslayer into a black hole in which neither could escape from.