Vaatu is the dark spirit that fuses with Unalaq to create a Dark Avatar. He represents darkness, chaos, and disorder, opposing Raava, the spirit of light and harmony.
In The Legend of Korra’s second season, Unalaq frees Vaatu from imprisonment beneath the Tree of Time. During Harmonic Convergence, Unalaq merges with him and becomes a powerful human-spirit hybrid. Their goal is to defeat Korra and usher in an age dominated by darkness.
Vaatu is not simply an evil version of the Avatar. He is Raava’s complementary counterpart, and the two spirits are locked in a cosmic cycle: each contains the potential for the other to return. Korra ultimately separates Raava from Vaatu and defeats Unalaq, although the conflict permanently changes the relationship between humans and spirits.