13. Darkness Falls

Complete story — includes the ending

Korra tries to imprison Vaatu while Mako and Bolin attempt to keep Unalaq out of the spirit world. Aided by Desna and Eska, Unalaq overpowers Mako and Bolin, ambushes Korra, and merges with Vaatu, becoming the Dark Avatar. Korra and Unalaq then battle each other in their respective Avatar States. In the Spirit World, Tenzin and his siblings meet Iroh, who indicates how to find Jinora. The siblings then venture into the Fog of Lost Souls, which drives humans mad with their self-doubts. Kya and Bumi succumb to the fog and Tenzin nearly does as well, but is visited by a vision of Aang, by whose advice Tenzin clears the fog and frees his relatives, accepted his destiny as separate from that of his father for the first time in his life. Mako and Bolin convince Eska and Desna to free them, but cannot render aid to Korra in time; Vaatu tears Raava out of Korra and Unalaq severs her connection with her past lives. Jinora senses the destruction of Raava and goes to help. Unalaq transforms into "Unavaatu", a powerful dark spirit, and departs into the physical world to destroy it and remake it in a fashion of his own choosing.