The Anti-Monitor Crisis, the World Wide Crisis, the Crisis of Infinite Worlds, or simply the Crisis and also known as the Crisis of 2390, was a massive, apocalyptic event that encompassed the cataclysmic destruction of every single Earth in the multiverse by the Anti-Monitor's wave of antimatter, leaving only the chosen Paragons as the last survivors.
Oliver Queen would later on become the Spectre, gathering the Paragons and reformed the multiverse and merged Earth-1, Earth-47 and Earth-38 to create Earth-47A before an alliance of heroic teams banished the Anti-Monitor to the microverse. This also resulted in the reformation of the Justice League in order to put down further threats to the new multiverse.
In the original timeline, the Crisis took place on the night of April 25, 2398, where both the Flash and his nemesis Reverse-Flash traveled back in time to March 18, 2371 to create the Flash, resulting in a temporal loop that allows the Crisis to happen in every timeline regardless, except the Flashpoint timeline.
Due to changes in the timeline, the Crisis occurred much earlier, on December 10, 2390.