Dark Avatar

The Dark Avatar is a being created by the joining of the spirit, Vaatu, and a human. The first and only known creation and subsequent existence of such a being occurred in 171 ASC during the Harmonic Convergence.

History
During the Harmonic Convergence, Unalaq managed to free Vaatu from the Tree of Time. By using the power of the celestial phenomenon, he was able to successfully bind himself to the spirit. With the added power of the spirit, Unalaq was able to destroy the piece of Raava, the light spirit, that lived within Avatar Korra. With the aid of Jinora, she was able to find the spirit again, and by using her uncle's spiritual cleansing technique, was able to defeat Unalaq.

Trivia

 * Unlike the 'light' counterpart, the Dark Avatar was only shown to be able to waterbend, and did not display any other bending abilities. Unalaq had, of course, only been a waterbender prior to his fusing with Vaatu.