Vaatu is the spirit of chaos and darkness in the world of Avatar: The Last Airbender. He is one of the oldest known spirits, having existed over ten thousand lifetimes before the appearance of the first humans and has likely been around since the dawn of time. He and Raava, the spirit of light and order, have spent eternity combating each other every Harmonic Convergence, with neither able to fully vanquish the other. Even if Vaatu were to destroy Raava and usher in a new era of peace, what little light existed in Vaatu would magnify until Raava was reborn and burst forth, beginning the cycle again. The reverse would happen to Raava if he were victorious over Vaatu.
He was the first spirit to cross over into the mortal world by breaking down the barrier that separated it from the plane of spirits, subsequently allowing other spirits to do the same. His essence swayed the spirits to the dark side, transforming them into dark spirits.
During the Harmonic Convergence in 9,829 BSC, Vaatu was defeated by a merger of Raava with a human named Wan, and subsequently locked inside the Tree of Time for ten thousand years. During the next Harmonic Convergence in 171 ASC, Vaatu permanently merged his essence with a corrupt human named Unalaq, creating the Dark Avatar Spirit. Their form, however, was purified by Avatar Korra at the Battle during Harmonic Convergence.