Vaatu

Vaatu is the spirit of darkness and chaos, 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.

When Korra arrived at the location of Vaatu's imprisonment, intending to close the Southern portal, Vaatu drew the young Avatar to him before she could complete her mission, addressing her as Raava and informing her that, upon the arrival of Harmonic Convergence, he would destroy her for good. As Korra walked away from him undeterred 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 as they were imbued with a rush of built-up spirit energy that had accumulated during the ten thousand years of the portal's closure. As the Avatar was carried away to safety by a dragon bird spirit, Vaatu proclaimed that he would see her once more upon being freed and would finally have his revenge.

When the Harmonic Convergence occurred not long after, Korra failed to close the Southern portal in time before the planets aligned. The resulting influx of spirit energy strengthened Vaatu, allowing him to break free from his millennia-long confinement in the Tree of Time. After taunting Raava about how the inevitability of his escape and the resumption of their ages-old conflict, he tried to merge with Unalaq, but his attempt was thwarted by Korra's firebending. After using her airbending to forcibly eject Unalaq from the Spirit World, she engaged in a solo duel with the Dark Spirit. During the ensuing battle, Vaatu was trapped inside an elemental cage once again, but before he could be locked back inside the hollow of the Tree of Time, Unalaq attacked Korra from behind, freeing Vaatu from her hold. With Korra no longer having the strength to fight back, the Dark Spirit merged his essence with Unalaq's, who promptly used the energy from Harmonic Convergence to make their fusion permanent, becoming the Dark Avatar.

Proclaiming to be the new Avatar who would lead the world in to a new era, they were challenged by Korra, who stated her era to not have been over yet. The two Avatars battled each other in a final duel for supremacy, the resulting fight spilling into the physical world. When locked in a show of strength, Vaatu reached out with a tendril from within Unalaq's throat and extracted Raava out of Korra, ripping their presumed permanent fusion apart and enabling Unalaq to destroy the Light Spirit with a barrage of water attacks, infused with his technique to corrupt a soul. With Raava out of the way and Korra effectively neutralized, the pendulum of balance swung completely in Vaatu's favor, allowing him to physically manifest his features through Unalaq, and the Dark Avatar grew into a massive red giant. Using the spirit lights, the Dark Avatar traveled to Republic City where he demolished the metropolis with spirit vines. In the midst of his triumph, however, Korra's astral projection unexpectedly attacked him; though after she failed to locate Raava's residual light inside of him, the Dark Avatar began to corrupt her spirit with spiritbending. Just as the Dark Avatar proclaimed his triumph once and for all, Jinora's spiritual projection brought Raava back into the world and illuminated the Light Spirit inside of Vaatu's essence, allowing Korra to extract Raava and subsequently spiritbend him, dissipating the Dark Avatar Spirit in a golden light.

Legacy
When Korra was administered a poison by the Red Lotus that would make her enter the Avatar State, Vaatu appeared to Korra in one of her hallucinations, having sprung forth from Ming-Hua. He floated up to Korra, telling her that she was too weak to resist the poison and that he was stronger than ever. Emphasizing the futility of her resistance, he urged her to just let go and enter the Avatar State.

Vaatu was later used by Varrick in his story of Bolin, Hero of the World, in which he was described as "the ultimate force of pure spirit evil in the universe; the biggest, meanest, scariest kite that ever flew". He teamed up with Amon, Unalaq, and Zaheer to take out their common nemesis, Bolin, for which he was added to a conference call by Zaheer. Upon being rung by the airbender, Zaheer joked that he was glad to have caught him at home, jabbing at the spirit's imprisonment in the Tree of Time, much to his dismay. After Unalaq entered the conversation as well, Vaatu and his two other companions tried to get rid of the Chief of the North and eventually just hung up on him. Vaatu was said to be freed from his prison inside the tree when Juji's lasers miss Unalaq and bounced off Vaatu instead. However, due to Unalaq's proximity, who was described as always having been an "annoyingly, clingy person", the two merged and formed UnaVaatu. In their giant forms, UnaVaatu and Bolin were reported to fight on par and barrage each other with laser beams while stomping around Republic City. Although UnaVaatu nearly managed to corrupt Bolin's soul with spiritbending, he was defeated by the combined power of Bolin and the queen of the fairies and turned into magic dust to be scattered across the sky to give birth to the stars.

Personality
Vaatu's personality is fairly two-dimensional, as he is consumed by his single-minded objective to destroy Raava and bring about ten thousand years of darkness upon the arrival of Harmonic Convergence. His apparent distaste for Raava is intrinsic to his very nature as her spiritual counterpart, though it is further exacerbated when Raava managed to keep him in check and later, after having merged with Wan, sealed him in the Tree of Time, fueling his desire for revenge. Apart from wanting to destroy Raava forever, he follows the similarly narrow path of spreading chaos and destruction, since these are the very things he personifies and give him strength.

Despite spreading chaos by default, Vaatu exudes an ironic calm, never losing focus of his goal or panicking, even in the face of certain defeat. He patiently bid his time in the Tree of Time for ten thousand years, waiting for someone to free him. He is confident in his abilities and his pride waxes into pure arrogance when he deals with humans, a race which he disdains, being far older and more powerful than they are, and thinking that no human could ever be a match for him. Even though Avatar Wan defeated him, he refused to acknowledge Wan's reincarnation, Korra, and only refers to Raava when speaking to her. The only human he seems to acknowledge was his human ally, Unalaq, whom he treats relatively fairly and without contempt, even when Unalaq believed that he had failed Vaatu. This fact further shows that, despite being an innately destructive being, Vaatu is coolly calculating, able to put aside his prejudice toward humans to suit his overall goals, even going so far as to merge with one to get even with Raava, turning her own strategy against her. Even in the beginning, while Raava was disdainful of Wan's interference, Vaatu was insightful enough in his desperation to manipulate Wan's love of spirits and trick him into freeing himself from Raava's grasp.

Abilities
As the spirit of chaos and darkness, Vaatu has the ability to magnify the negative emotions inside other spirits, swaying them to give in to the darker sides of their natures. In turn, he feeds on chaos and anarchy, increasing in both size and power. His airy and transparent body allows him to fly and be unharmed when it is severed by an attack, though it has enough substance to enter physical combat and produce kinetic force. He also has the ability to shoot purple energy blasts from his "face". In his battle with Korra, and later in his assault on Republic City, he also demonstrated the ability to generate vegetation infused with spiritual energy that can take a life of its own by shoving his tentacles into the earth and making vines and other plants grow.

Like his counterpart, Raava, Vaatu is effectively immortal, having lived for tens of thousands of years. If he is overcome by Raava, he can gradually reform himself from the vestigial darkness within her. Though they have been separated for ten thousand years, Vaatu still has a connection with Raava, and was able to sense her presence once Korra entered the Spirit World. Vaatu is also fully capable of binding himself with a human being, which was exemplified during the Harmonic Convergence of 171 AG as he merged with Unalaq, thereby producing the Dark Avatar. This merge also allowed Vaatu to gain control of Unalaq's ability to waterbend, though unlike Raava, Vaatu's presence in Unalaq's body manifested itself on the outside, as shown by the physical transformation of the Dark Avatar upon Raava's defeat whereby said transformation resembled Vaatu substantially. His presence within Unalaq exhibited more sentience than Raava's in the Avatar Cycle, as demonstrated by him and Unalaq speaking in unison as well as his taking control of Unalaq's body in order to extract Raava from Korra.

Appearances
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Book Two: Spirits (神靈)

 * 207. "Beginnings, Part 1" (vision)
 * 208. "Beginnings, Part 2" (vision)
 * 209. "The Guide"
 * 210. "A New Spiritual Age"
 * 212. "Harmonic Convergence"
 * 213. "Darkness Falls"
 * 214. "Light in the Dark"

Book Three: Change (易)

 * 313. "Venom of the Red Lotus" (vision)

Book Four: Balance (平衡)

 * 404. "The Calling" (vision, no lines)
 * 408. "Remembrances" (vision)
 * }
 * }

Trivia

 * Vaatu and Raava appear respectively to be representations of the yin-yang (Taìjí tú) concept in Chinese philosophy, which is used to describe the way in which opposite forces are interconnected and interdependent in the natural world. In this case, the concept accounts for the duality associated with the fact that Vaatu represents darkness and chaos, while Raava represents light and peace. His attributes, however, are ironically closer in Taoist thought to the ideal of light (yang) than that of darkness (yin); Vaatu is male and aggressive, as opposed to female and calm.
 * Furthering the yin-yang comparison, Vaatu and Raava are said to contain vestigial elements of each other within themselves, from which they can regenerate after being defeated. Similarly, each side of the yin-yang symbol bears a small dot of the opposite color, symbolizing how each side bears a part of the other.
 * This duality is also apparent in their names and color schemes. In Sanskrit, vatu (वतु) is an interjection meaning "silence!", which is the opposite of the noun ravaḥ (रवः), which means "sound". The two spirits are also negative images of each other.
 * Vaatu and Raava are also comparable to the Zoroastrian concept of moral dualism, Vaatu being similar to Ahriman/Angra Mainyu in representing darkness and immorality.
 * As Vaatu grew stronger, he became more detailed in his pattern and gained more tendrils.
 * Vaatu derisively addressed the Avatar as Raava. Ironically, Korra defeated him on her own, without Raava, while Vaatu was the one merged with a human.
 * By utilizing Vaatu's dark power, Unalaq was able to corrupt and force spirits to fight Korra during Harmonic Convergence.