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, creating the Dark Avatar Spirit. Their form, however, was purified by Avatar Korra during the Battle during Harmonic Convergence.
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 he would destroy 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 off of 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.
The Dark AvatarEdit
Ten thousand years later, near the next Harmonic Convergence, Vaatu was visited by Unalaq, who tried to free him. After the Water Tribe chief failed to open the Northern spirit portal, Vaatu assured the waterbender that all was not lost yet, as he could sense Raava's presence in Korra. The spirit of darkness told Unalaq that his niece would find him.
When Korra arrived at the location of his imprisonment intending to close the Southern portal, Vaatu drew the young Avatar to him before she could complete her mission, and addressed her as Raava, informing her that, upon the arrival of the Harmonic Convergence, he would destroy her for good. As an undeterred Korra walked away from him to close the portal, Vaatu suggested that she reconsider if she wanted to save Jinora, who had been captured by Unalaq. When Korra subsequently opened the Northern portal, Vaatu and the tree in which he had been imprisoned began to glow, and as the Avatar was carried away to safety by a dragon bird spirit, Vaatu proclaimed to the spirit of Raava that he would see her once more upon being freed by the alignment of the planets, and would finally have his revenge.
When the Harmonic Convergence occurred in 171 AG, Vaatu broke free from the Tree of Time. He instantly tried to merge with Unalaq, but his attempt was thwarted when Korra firebent at him and subsequently used her airbending to expel Unalaq from the Spirit World. During the ensuing battle, Vaatu was trapped inside an elemental cage once again, though before he could be locked back inside the hollow of the Tree of Time, Unalaq attacked Korra, freeing Vaatu from her hold. The Dark Spirit merged his essence with Unalaq's and Unalaq used the energy of the Harmonic Convergence to make their fusion permanent, creating the Dark Avatar Spirit.
Together, they challenged Korra, their fight extending to the mortal world. When locked in a show of strength, Vaatu reappeared and extracted Raava out of Korra, enabling Unalaq to subsequently destroy the Light Spirit. With Raava out of the way and Korra neutralized, Vaatu manifested his features through Unalaq, and the Dark Avatar grew to massive proportions. Using the spirit lights, the Dark Avatar traveled to Republic City where he laid waste to the metropolis. Korra's astral projection challenged him, though after she failed to locate Raava's residual light inside of him, the Dark Avatar tried to corrupt her spirit with Unalaq's purification technique. However, when Jinora's spirit brought Raava back into the world and illuminated the Light Spirit inside of him, Korra was able to extract Raava from inside of him and subsequently purify him, dissipating the Dark Avatar Spirit in a golden light.