The Crisis on Infinite Earths was a Multiversal event that resulted in the destruction of approximately 600,000 universes, including Earth-Prime and Megacrossover Earth. It was caused by a being from an antimatter universe called the Anti-Monitor who sought to destroy all worlds made out of matter. Several heroes from the remaining Earths united to stop the Anti-Matter at the dawn of time. Their presence created a new universe that was a combination of their home realities that is known on Earth-Prime as the DC Universe.
Shortly after the Anti-Monitor was defeated, Eru restored the destroyed universes. All involved beings had their memories wiped or altered except for the Elder Council, the Archdevils, and few other selected individuals. Heroes that died were restored in their home realities, but were absent in the new universe.