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. It's revealed that he is the true main villain of Book 2 and one of the foremost antagonists of the Avatar Franchise.

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.

However, as of The Legend of Korra, Vaatu has made efforts to bring to about his return, corrupting Spirits into Dark Spirits and also enlisting the help of Unalaq to free him. Unalaq has convinced Korra to open both the Northern Portal and the Southern Portal, and so upon Harmonic Convergence, Vaatu not only escapes, but also merges with Unalaq to become the first Dark Avatar.

Unalaq/Vaatu proceed to attack the Avatar, rip Raava out of Korra, destroy Raava (temporarily, as one cannot exist without the other), and attack Republic City. When Korra faces them a second time, Jinora's spirit pulls a small light out of Unalaq/Vaatu, recreating Raava. Raava and Korra merge once again, performing the spirit purification ritual in Unalaq/Vaatu, causing them to vanish.

Since one cannot exist without the other, Vaatu will eventually be recreated the same way Raava was. His host (Unalaq), however, remains deceased.

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.
 * He bears some similarities to Hexxus from Ferngully. Both are shadowy monsters possessed of apparent immortality who though locked away for centuries inside a tree eventually return to bring ruin and destruction to the world once more, as well as coming into a conflict with a female hero and taking sadistic joy in defiling and corrupting things that were once good (for Hexxus nature and for Vaatu Spirits). Finally, neither one can be outright destroyed for good, only imprisoned or otherwise prevented from doing more harm.
 * Vaatu's backstory and symbolism quite reminiscent with Tathamet's. Much like Vaatu and Raava, Tathamet was trapped in a brutal conflict with Anu. Both Vaatu and Tathamet represent Chaos and Darkness. But unlike Vaatu, who is eventually sealed inside a tree by Raava and the first Avatar before freed again, Tathamet and Anu destroyed each other, which none of them survives. In addition, Tathamet is also born from Anu's dark aspect, whereas Vaatu and Raava are separate entities.