Vaatu is the spirit of chaos and darkness, who briefly became the Dark Avatar Spirit during the Harmonic Convergence of 171 AG. He is one of the oldest known spirits, having existed over ten thousand lifetimes before the appearance of the first humans. He was the first spirit to cross over into the mortal world, breaking down the barrier that separated it from the plane of spirits and creating the Northern and Southern spirit portals in the process, enabling other spirits to do the same. His essence swayed the spirits to the dark side, transforming them into dark spirits.
Vaatu and Raava, the spirit of peace and light, spent much of their existence combating each other, at least since the Harmonic Convergence of 19,829 BG, with neither able to fully vanquish the other. Even if Vaatu were to destroy Raava, what little light there was in him would magnify until a reborn Raava burst forth, beginning the cycle again. The reverse would happen to Raava if she were victorious over Vaatu.During the Harmonic Convergence in 9,829 BG, Vaatu was defeated by a merger of Wan with Raava, and subsequently locked inside the Tree of Time for ten thousand years. During the next Harmonic Convergence in 171 AG, Vaatu permanently merged his essence with Unalaq's, becoming the Dark Avatar Spirit. Their form, however, was purified by Avatar Korra during their battle. As a result, Vaatu receded into Raava and the Avatar, where he will continue to grow stronger and eventually break free.