Zero Hour was a DC event that sought to deal with the leftover continuity problems from the Crisis on Infinite Earths. Problems with the timestream aggravated by Parrallax and Extant caused denizens of various alternate Earths to appear on the main DC Earth.
9 worlds
Zero Hour
Earth: Alfred is a detective
Batman: Shadow of the Bat #31
Earth: Alpha Centurion
Adventures of Superman #516
Earth: Bruce Wayne killed instead of parents
Detective #678
Earth: Green Arrow dies
Green Arrow #90
Earth: Israeli Hawkman
Hawkman #13
Earth: Joker kills Commissioner Gordon, Batgirl not shot
