Imperiex Prime is the embodiment of Entropy, Entropy is the most powerful form of power ever known. It is the end of everything. Entropy (Imperiex) was let into the Precrisis/Postcrisis DCU after the Anti-Monitor tried to destroy the Universe. Entropy is also a being in Marvel which is suppose to be Eternitys young son, Entropy is suppose to someday replace/destroy Eternity.
Imperiex Prime said he was "Many times older then the current pre/post crisis Universe". He has destroyed the DCU already a few times (something Anti-Monitor failed to do.). He was a threat not just to 1 timeline but to every timeline in Hypertime.
He was hollowing the entire DCU destroying key planets to destroy everything with a few super black holes (which is a lot bigger then regular black holes).