All of the Supermen were taken to a central spacecraft, where they, along with the various Doomsday analogues (inc. the original Doomsday) were being held:
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After saving the original Doomsday, Eradicator inhabited his body by accident. We then saw that the original version is superior to the other knockoffs (as powerful as they were):
Turns out that "Doomslayer" (a HIGHLY evolved version of Doomsday) was responsible for the armada of Doomsday analogues that were running around owning the heroes. Doomslayer himself was also quite powerful...
Kills Eradicator:
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Owns Superman with a casual backhand:
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Owns him again with energy beams:
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Doomslayer's plan was to destroy ALL Doomsdays. To do this he was willing to destroy the universe: