“Crisis on Infinite Earths,” written by Marv Wolfman and drawn by George Perez, was one of the first major crossovers, and it dramatically changed the face of the DC Universe. You see, for years, there were different Earths, each one having their own heroes. Earth-1 had the heroes introduced in the Silver Age. Earth-2 had the heroes introduced in the Golden Age. Earth-X had the heroes DC Comics had purchased from Quality Comics. So on and so forth. Rather than a single universe, DC had a multiverse.Well, in the multiverse, there were two powerful beings watching everything – a Monitor and his parallel – an Anti-Monitor. The Anti-Monitor wanted to destroy the entire Multiverse. The Monitor gathered together a number of superheroes to help save the multiverse, which was slowly being destroyed by the Anti-Monitor – one Earth at a time.
In the ensuing conflict, more and more heroes got involved, and the Monitor himself was killed. In one early battle, Superman's young cousin, Supergirl, died fighting the Anti-Monitor. Her death managed to cause the Anti-Monitor to retreat, leaving five Earths left in the multiverse to fight against the Anti-Monitor.
Later, in an attempt to stop the Anti-Monitor from using an anti-matter cannon to destroy the remaining five Earths, the Silver Age Flash, Barry Allen, raced so fast that he turned into a beam of energy that stopped the cannon.Eventually, through the intervention of the Spectre, one combined Earth remained. The Anti-Monitor was not done yet, though, as he transported this last Earth to an anti-matter universe in one last attempt to destroy the Earth. This took place in the last issue of Crisis on Infinite Earths. A gigantic assortment of heroes led by the Superman of Earth-2, Alexander Luthor of Earth-3 and the Superboy of Earth-Prime managed to finally defeat the Anti-Monitor (with an assist from the villainous Darkseid, of all people!) and sent him into a star, killing him, saving the last remaining Earth.