Vaatu

"See you at the end of the world"

- Vaatu to Wan

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.

History
Vaatu first appeared engaged in battle with his opposite number Raava. Attracting the attention of a then wandering Wan, Vaatu convinced Wan that Raava was the instigator of the conflict and was oppressing him. Falling for his ruse, Wan freed Vaatu from Raava's grasp, allowing him to flee. Once freed, Vaatu wasted little time corrupting Spirits and turning them into Dark Spirits, also planning to cause 10,000 years of darkness on Earth.

Wan, realizing his mistake, tried to help Raava defeat Vaatu and the two traveled the world, with Wan learning all four elements so as to better help Raava fight Vaatu. With Raava's help, Wan was able to master them all. Unfortunately during that time, Vaatu had been harnessing the Spirit's anger to become stronger while simultaneously making Raava weaker, also creating more and more Dark Spirits.

Raava explained that in a year, Vaatu would use the Harmonic Convergence to make Raava disappear forever and allow him full power. When the time of Harmonic Convergence came, Wan engaged Vaatu in battle, but Vaatu was far too powerful to be defeated. Raava entered Wan's body and the fight resumed, only for Vaatu to triumph anyway. At that moment the Harmonic Convergence occured but before Vaatu could complete his plan, Wan used the Harmonic Convergence to merge Raava with his soul forever, making Wan the very first Avatar. Wan was able to then seal Vaatu in a tree and sealed away the Spirit World, putting an end to Vaatu's threat for 10,000 years.

Trivia

 * Vaatu's backstory, role, and physical appearance bear some resemblance to Aku, who's voice actor Mako also voiced another Avatar character, Iroh.
 * He is voiced by Jonathan Adams, who has voiced several other villains, including Dormammu, who Vaatu is similar to.