|"When Harmonic Convergence comes, I will fuse with Vaatu, and together we will become the new Avatar. A Dark Avatar."|
|— Unalaq to Korra.|
The Dark Avatar, or UnaVaatu in its enlarged form, was the antithesis of the Avatar and was created from the fusion of Vaatu and Unalaq, forming an embodiment of darkness and chaos. The Dark Avatar aimed to reunite the physical and spiritual realms as well as usher in a new world order of ten thousand years of darkness, but was defeated and killed by Korra, as an astral projection, during the Harmonic Convergence of 171 AG. With his spirit purified, the Dark Avatar Cycle was ended before it began, although Vaatu will eventually regenerate within Raava.
After joining the Red Lotus, a young Unalaq learned about Raava and Vaatu, and the history of how the Avatar came into existence. Unalaq believed that humans and spirits should co-exist despite there being an Avatar, contrary to Wan's idea to separate the two species by closing the spirit portals. As time went by, Unalaq came up with a plan to achieve his idea, which was to free Vaatu during the next Harmonic Convergence and fuse with him in order to become a Dark Avatar, a plan that went contrary to the Red Lotus' goals of destroying the Avatar and the Avatar Cycle altogether. He would later betray the organization in order to pursue his idea, covering his tracks and conspiring with Vaatu to pursue this goal, eventually manipulating Avatar Korra into opening the spirit portals in order for Vaatu to free himself.
Following Vaatu's escape from the Tree of Time during the Harmonic Convergence of 171 AG, the dark spirit merged with Unalaq and permanently bound himself to the waterbender's spirit using the energy provided by the overlap of the two spirit portals. This process created the Dark Avatar, who promptly began to battle the Avatar for the fate of the world. Despite a strong showing from both Avatars, Unalaq managed to subdue Korra, at which point Vaatu emerged from his mouth, latched onto Korra's face and pulled Raava out of Korra's body. With the use of his waterbending, enhanced with Vaatu's dark energy, Unalaq destroyed Raava, severing Korra's connection to her past lives in the process. Having defeated Raava, Unalaq and Vaatu grew exponentially more powerful, which was displayed by the Dark Avatar growing significantly in size to a height several feet tall, including a physical metamorphosis.
After declaring the era of the Avatar to be over, the Dark Avatar used the spirit lights to transport himself to Republic City, where he promptly proceeded to begin wreaking havoc on the city's infrastructure using vine-like projections. Despite heavy bombardment by the United Forces, the Dark Avatar was able to use augmented waterbending to scatter the battleships as well as knock the lead police airship and a formation of biplanes from the sky, prompting President Raiko and Chief Lin Beifong to evacuate.
Shortly after, a large astral projection of Korra arrived and began to battle the Dark Avatar. Using cosmic energy, Korra was able to overwhelm the Dark Avatar and attempted to extract light from within Unalaq, to no avail. This setback allowed the Dark Avatar to subdue Korra with a dark variation of spiritbending and nearly corrupted her spirit, until Jinora's spirit appeared to intervene. The young airbender, whose own spiritual powers were augmented by the Convergence, released a beam of light, allowing Korra to find Raava within the Dark Avatar and subsequently remove her before cleansing the Dark Avatar using her uncle's own purification technique, destroying the Dark Avatar in the process.
Unlike the Avatar, the Dark Avatar did not have the ability to bend all four bending arts, since Unalaq could only waterbend prior to his fusion with Vaatu, unlike Wan, who went to the lion turtles with Raava and received all four elements.
Dark Avatar State
Despite the limitation of only bending one element, the Dark Avatar's bending potential could still be significantly augmented by a dark variation of the Avatar State, in which Vaatu provided Unalaq an energy surge which allowed him to accomplish great feats of waterbending, such as creating large waves capable of dispersing several battleships.
In this state, Unalaq's eyes glowed orange-red, as opposed to the white glow of the Avatar's eyes when they entered the Avatar State. Due to being fully aware of his spiritual capabilities, Unalaq had full control over the Dark Avatar State the moment he became the Dark Avatar and he was able to trigger the ability with ease whenever he found it appropriate to use.
After vanquishing Raava, the Dark Avatar underwent a physical transformation, becoming a massive giant several feet tall, with enhanced powers and physical attributes from both Unalaq and Vaatu. However, despite his humanoid appearance, the Dark Avatar's new form bore a strong resemblance to Vaatu's spiritual form, retaining his tentacle-like appendages. His skin color and body markings resembled those of Vaatu's spiritual form as well. Being that the two were so closely connected in this form, this extreme fusion is termed "UnaVaatu". Unalaq's waterbending was far more powerful in this form than in his Dark Avatar State, with UnaVaatu able to create a giant wave proportionate to his size that easily defeated the United Forces fleet. UnaVaatu was also able to spiritbend on a larger scale, which he used against Korra's astral projection. Furthermore, UnaVaatu possessed several of Vaatu's abilities that the Dark Avatar had never demonstrated, such as fighting with tendrils and shooting energy beams from the markings of his chest.
- Vaatu displayed a greater degree of sentience in his fusion with the new Dark Avatar as compared to Raava and the respective Avatars. This was demonstrated most prominently during the fight with Korra, where he took control of Unalaq's body, reached out from within him and ripped Raava out of Korra, after which Unalaq regained control of his body. Additionally, his greater sentience was demonstrated by his voice speaking simultaneously with Unalaq's upon the Dark Avatar's evolution post-destruction of Raava.