Vaatu

Vaatu was the Spirit of Chaos and Darkness that appeared in the two part episode Beginnings of ''The Legend of Korra. ''He fought the first Avatar Wan 10,000 years ago and attempted to destroy the world. Vaatu could be the Avatar franchise's equivalent of Satan.

History
When Wan saw Raava and Vaatu battling, he separated the two since their fight was causing destruction and believed Vaatu was being bullied. Once freed, Raava tells Wan that Vaatu planned to cause 10,000 years of darkness to Earth. Wan, realizing his mistake, tried to help Raava defeat Vaatu and the two traveled to the last three Lion Turtles. With Raava's help, Wan was able to master all four elements. Wan then engaged Vaatu in battle, but Vaatu was far to powerful to be defeated. Raava entered Wan's body and after the Harmonic Convergence Vaatu planned to use to conquer the world, merged with his spirit forever, making Wan the very first Avatar. Wan was able to seal Vaatu in a tree and sealed away the Spirit World, putting an end to Vaatu's threat for 10,000 years.