Crisis on Infinite Earths
The Crisis on Infinite Earths was a reality-changing series of events centered around the efforts of the Anti-Monitor to destroy the matter multiverse and thus attain ultimate power. In the wake of the event (later known as simply "the Crisis"), thousands of universes, and all the people and things within them, were erased, and many heroes and villains from the worlds that survived were wounded, irrevocably changed or outright slain. In the end, the remaining five universes- Earth-1, Earth-2, Earth-4, Earth-S and Earth-X- were merged into a single reality ("Earth-Sigma") with a fixed past and future (or so was said), and the Anti-Monitor himself finally destroyed.
After the Crisis reorganized reality, the event itself was rewritten to be the Anti-Monitor's attack on time, where he hoped to force all history to merge into a single, simultaneous point, in order to destroy the entire matter universe in a single blow. The true events of the Crisis remained known to only a select few, such as the Time Trapper and the Linear Men.
More specifics on the twists and turns of the Crisis can be found in the Anti-Monitor's entry.