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.
In the ten thousand years since the Harmonic Convergence immediately before Wan's time, Vaatu had been in conflict with Raava and was restrained by his light counterpart. However, when Wan arrived during their battle, Vaatu seized his opportunity and tricked Wan into separating him from Raava. Free to roam the world, he spread chaos wherever he went, growing stronger and weakening the light in the process.
During his travels around the world, he discovered a hidden village of men atop an airbending lion turtle. His presence transformed the peaceful spirits living there into rageful entities. However, Wan and Raava's intervention prevented him from laying waste to the village. He taunted Raava over growing weaker due to his proliferation of chaos and threatened to destroy her forever during the upcoming Harmonic Convergence, which would take place a year later.
Vaatu met with Raava and Wan once more at another conflict between spirits and humans. Vaatu turned the spirits' anger against them, turning them into dark spirits. After the battle was over, Vaatu told Wan that the humans had been slain and that he should enjoy his last days before the end of the world.
As Harmonic Convergence drew near, Vaatu used the Northern spirit portal to travel to the Spirit World where he was set to battle Raava for supremacy. However, it was not the light spirit alone that he faced, as Wan had allied himself with her. Vaatu was challenged by Wan, though he brushed away the threat in the challenge, claiming that no human could stand against him. A battle between the two ensued, during which Vaatu had the upper hand. When he was about to kill Wan, the human merged with Raava, and Wan managed to escape Vaatu for a moment. However, the dark spirit managed to beat his merged opponents down, pinning them to the ground next to the Southern spirit portal at the moment Harmonic Convergence began.
Vaatu looked up at the overlapping of the spirit portals, taking his focus away from Wan, who used that time to merge permanently with Raava by touching one of the portals near him, amplifying his spiritual energy. In the ensuing battle, Vaatu was trapped by Wan in a sphere of air that was surrounded by the four elements. Captured in this elemental cage, he was forced inside the hollow of the Tree of Time where he would remain restrained. With Vaatu's influence neutralized, the spirits he turned dark were able to regain balance within themselves. To ensure Vaatu's imprisonment for eternity, Wan closed both the Northern and Southern spirit portals so no human would ever be able to physically enter the Spirit World again and release Vaatu.
Creating the Dark Avatar
Ten thousand years later, shortly before the next Harmonic Convergence in 171 AG, Vaatu was visited by Unalaq, who tried to free him. After the Water Tribe chief failed to open the Northern spirit portal without the Avatar's help, whom he believed to be dead, Vaatu assured the waterbender that all was not lost yet, as he could sense Raava's presence in the Avatar. The Spirit of Darkness told him that Korra would find him, as she had entered the Spirit World while they were talking.