Harmonic Convergence

Harmonic Convergence is a periodic event that happens once every 10000 years. During a planetary alignment, Raava and Vaatu battle each other in the Spirit World, near the spirit portals that lead to the physical world. Whichever one wins generally determines how the world will fare until the next Harmonic Convergence. Raava and Vaatu cannot permanently destroy each other, as the loser of the fight is eventually regenerated from the other. During Harmonic Convergence, the spirit portals connect through the spirit world, causing a reaction in the two spirits.

History
During the last Harmonic Convergence, Raava was at a disadvantage, due to Wan's recent interference with her restraining of Vaatu. As Vaatu was let loose into the world, his powers grew, while Raava became weaker. To atone for his mistake, Wan put himself at Raava's service, and with her help, collected the power of multiple elements, to aid her in fighting Vaatu. Despite a difficult start, Wan and Raava's joint effort was enough to incapacitate Vaatu, who was imprisoned inside a tree in the Spirit World. To prevent humans from freeing him again, Wan led the spirits from the physical world into the Spirit World, and closed the spirit portals.

Ten millennia later, Avatar Korra learned about the origins of the Avatar and the Harmonic Convergence from Wan while recovering from a dark spirit attack that had left her amnesiac. Upon regaining her memory, Korra spoke with the shaman that had aided, and discovered that the next Harmonic Convergence was only weeks away. Determined to close the southern spirit portal, Korra set out on a flying bison from the shaman's herd.