Crisis on Infinite Earths

From Superdickery

Quite possibly the largest crossover in DC comics history, the Crisis on Infinite Earths was DC's ultimate attempt to destroy confusing Silver Age continuity and restarting everything by having the evil villian the Anti-Monitor destroy the multiverse in order to rule what was left of the universe. He failed, but along the way the Anti-Monitor killed a buttload of old heroes, including the original Supergirl.

Unfortunately, the Crisis didn't work as well as DC hoped, leading to more retcon mega events such as Zero Hour, the most recent being the unimagitevely named Infinite Crisis.