To celebrate its 50th anniversary, in 1985 DC Comics published the 12-issue limited series Crisis on Infinite Earths. The series featured appearances by nearly every DC Comics character published and told a story that allowed the company, at the end of the series, to 'reboot' its entire line of comics: a cosmic battle ending with the recreation of the comics universe from the dawn of time with a single universe: New Earth. This allowed DC to launch a new era with a reinvention of its major character franchises, such as Superman, Batman, and Wonder Woman.
One by one, a villain known as the Anti-Monitor destroyed several alternate universes. Heroes of the last five universes (Earth-One, Earth-Two, Earth-Four, Earth-S, and Earth-X) along with a handful of survivors from other universes (Pariah, Lady Quark, Alexander Luthor, Jr.) held off the destruction of the last five universes long enough to defeat the Anti-Monitor.
After this, during a time travel incident, the Big Bang was altered, and only one universe/earth was created. In the modern day, this new DC Universe combined elements of the last five universes, along with completely new elements. For example, there was a Flash named Jay Garrick who was a member of the Justice Society during the 1940s, and another Flash named Barry Allen who was a member of the Justice League decades later, but there was only one Superman, who had a modified history, different in some respects from both the Earth-One and the Earth-Two versions.
Several pre-Crisis characters (most importantly the Kara Zor-El Supergirl and Barry Allen) were killed during Crisis on Infinite Earths, and as a result were either erased from history, as in Supergirl's case, or simply proclaimed dead, as in Barry Allen's case, in the new singular universe/earth. Wonder Woman was thought to have been slain in the final issue, but was revealed to have been thrown backwards through time, reverting to the clay from which she was formed. This set the stage for her reintroduction into the reformed DC Universe and the relaunch of the Wonder Woman comic, helmed by George Pérez. Other characters and concepts, such as Streaky the Supercat, Comet the Super-Horse, and the Space Canine Patrol Agents vanished without explanation.
Initially, most of the superhero characters remembered the Multiverse, but after some time their memories acclimated to the new timeline, and the pre-Crisis Multiverse was forgotten, except for only one character who remembered it: Psycho-Pirate. The final page of the story shows the Psycho-Pirate, who was now imprisoned in Arkham Asylum, talking to himself in a monologue: