Mageddon exists to cause life forms to fight against one another and is quoted as being more akin to a force of nature than a sentient being.
World War III
Mageddon crawls across the Multiverse, turning all living things against each other in a murderous frenzy through its prescence alone. When it destroyed Wonderworld - the realm of a group of cosmic beings similar to the Justice League - their equivalent to the Flash hid among the timestream, formulating a strategy to stop it.
When Mageddon approached Earth, its presence caused worldwide violence. Several mediocre villains were suddenly strong enough to break out of prison, while smarter villains such as Lex Luthor were possessed and acted as Mageddon's avatars. Superman, Orion and a new hero named Aztek all rose up to counter the beast, but upon infiltrating Mageddon's innards, were confronted by armies of robot drones. Superman approached Mageddon's nerve centre and attempted to break it, but was overwhelmed with a sense of dread that Superman had never felt before.
After the Justice League managed to calm the human population, the Flash met up with his Wonderworld counterpart and they constructed a machine that would give every single person temporary superpowers. Unfortunately, Mageddon was still to powerful for them and managed to wipe out a significant portion of the human population. Martian Manhunter probed Superman's mind, linking him up to Batman, the one man who could handle all the grief that Superman was feeling. Superman was able to come recover via Batman's improvised counselling and attacked Mageddon's core, destroying it.
Mageddon's corrupting influence might have started long before his earliest mention in the DC Universe. Given that many supervillains such as the Joker became increasingly violent for no apparent reason, it could be said that Mageddon is the harbinger of the Dark Age of comics.