Complex sentence: A single universe is created and all the superheroes return to a present-day reality where the various elements of the five Earths were fused into one single Earth, with no one except the people present at the dawn of time remembering the original reality.
Split into simpler sentences: A single universe is created and all the superheroes return to a present-day reality, in which the elements of the five Earths are fused into a single Earth. No-one except the people present at the dawn of time remember the original reality.