We are the Flood. There is no difference. Until all space and time are rolled up and life is crushed in the folds... no end to war, grief, or pain. In a hundred and one thousand centuries... unity again, and wisdom. Until then — sweetness.
~ The Primordial regarding the Flood's motivation for consuming intelligent life
Everything they've built will fall! And from the ashes of their world, we'll build a better one!
~ Apocalypse while recruiting Magneto to his ranks

A Harbinger for Rebirth is a subcategory of Cataclysm, in which the villain is not content with just destroying the entire world, universe or realm, but to '"revitalize" it, i.e. to have a "fresh start." They usually believe that humanity (or the current population of the realm, such as the Rats in Flushed Away whom the Toad wants to eliminate), as a whole, is beyond redemption and thus deserves nothing but destruction.

The motives for rebirth are far and wide, such as to obtain absolute power over the Universe or to remake it in his own image (the Dark One), to doom humanity for all time (as in the case of Sephiran), or as a twisted hope for redemption (as in the case of Shunack).