Unavaatu

Unavaatu is a Slash ship between Unalaq and Vaatu in The Legend of Korra series.

Canon
It is unknown how exactly Unalaq first came into contact with Vaatu but he learned about him while most of the world had forgotten about him, Raava and Wan. Unalaq soon decided that the Spirit World should not be cut off from the physical world and that Vaatu should not be trapped in the Tree of Time. Unalaq ended up tricking his niece, Avatar Korra into opening the spirit portals. Unalaq began to walk into the Spirit World in secret to speak with Vaatu.

When one of the spirit portals was open, Korra was believed to be dead before the other was open. Unalaq tried to get his children to open the other one with him but it did not work. Unalaq apologized to Vaatu and told him that he failed him. However, Vaatu reassured him and told Unalaq that he had not failed because Korra was still alive and in the Spirit World. Unalaq followed Vaatu's guidance to capture Jinora and force Korra to open the other portal.

Unalaq later revealed that his true plan was to use the coming Harmonic Convergence to permanently used with Vaatu and become a Dark Avatar. Vaatu was on board with this. Vaatu managed to escape from the Tree of Time, and although Korra tried to keep them apart, the two eventually combined and became the Dark Avatar. The two were very powerful and even managed to extract Raava from Korra and kill her. Afterwards, they went to attack Republic City until Korra arrived. She and Jinora eventually managed to bring Raava back from inside Vaatu and cleanse them with spirit healing. Since they were permanently combined, Vaatu and Unalaq died together.

Fanon
Unavaatu is a fairly popular ship in the Legend of Korra fandom. It gained a following as soon as it was revealed that they were working together and became even more popular after they fused to become the Dark Avatar. Some ship this as a parallel to Waava since both ships involved one working with a spirit and the two eventually being combined permanently.

Trivia

 * "Unavaatu" also refers to the gigantic form of the Dark Avatar.