The Legion of Doom is a group of supervillains who originated in Challenge of the Superfriends, an animated series from Hanna-Barbera based on DC Comics' Justice League.
[1] The Legion of Doom has since been incorporated into the main DC Universe, appearing in comics, as well as further animated and live-action adaptations, and also video games.
The episode "History of Doom" showed that Lex Luthor assembled 12 supervillains in order to form the most powerful and sinister group the world had ever seen.
When the Challenge of the Superfriends season was originally conceived, it was named Battle of the Superheroes and featured the introduction of Captain Marvel to the Super Friends.
Members of the Legion of Doom made some appearances in the Super Friends spin-off comic title based on the TV series: One of the nicest bits in "Alive!"