Raava and Vaatu are natural enemies. Raava is a spirit of light and order who wishes to protect life and maintain balance in the world while Vaatu is a spirit of darkness and chaos who wishes to spread chaos and cover the world in darkness. The two are always fighting and have a final battle that takes place during Harmonic Convergence ever ten-thousand years. When one dies, the one who does die will eventually be reborn inside of the other spirit, but this is a gradual process that happens over ten-thousand years.
Raava and Vaatu were eventually fighting in the Spirit Wilds and the former had the upper-hand. However, everything changed when a human named Wan arrived and was tricked by Vaatu into separating them. Raava made it her goal to chase him down so that she could stop him from spreading darkness and destroying all of humanity. Eventually, Wan decided to help her. At Harmonic Convergence, the two were losing badly until they permanently fused. After fusing, Wan and Raava managed to seal Vaatu into the Tree of Time. He would be trapped in there for ten-thousand years.
Ten-thousand years later, Avatar Korra was tricked into opening the Spirit Portals before Harmonic Convergence, which would allow Vaatu to escape. Korra ended up having amnesia and had to connect to Raava, who showed her the truth about Vaatu. Vaatu managed to escape and fuse with Korra's uncle Unalaq. Korra was about to be crushed in a fissure until Raava spoke to her and told her that Vaatu cannot win. After Korra escaped, Vaatu managed to extract Raava from Korra and Unalaq killed her. However, with the help of Jinora, Korra was able to pull Raava out of Vaatu which enabled her to defeat him.
Raavaatu is a fairly popular ship among the Legend of Korra. Despite the fact that they are eternal enemies, many ship them together because of balance and the fact that they represent yin and yang. The fact that they cannot truly exist without each other also fuels this ship. A lot fans like to portray Raava and Vaatu in humanoid forms and some do so while creating fanon for this ship.
- Vaatu and Raava are supposed to represent yin and yang, although the genders are reversed.