With Vaatu freed, Korra enters the Avatar State to do battle with the spirit, but not before she is able to toss Unalaq back into the material world. Mako and Bolin return to the South Pole to hold him off, while Tenzin, Bumi, and Kya go off to search for Jinora.
Bolin and Mako are able to hold off Unalaq successfully until he unleashes his children on him - much to Bolin's horror. Desna and Eska freeze the brothers as their father returns to the Spirit World. Like Wan before her, Korra has managed to overpower Vaatu and is about to imprison him again — but Unalaq arrives and stops her from doing so. Vaatu then merges with Unalaq, forming the Dark Avatar Spirit with Unalaq as the first Dark Avatar.
As the Avatars clash, Tenzin is having no luck in finding Jinora. To his surprise, Iroh appears to him, and offers to guide him and his siblings back to the material world. Tenzin refuses and reports Jinora's predicament, and Iroh gives him a cryptic clue to her whereabouts. Tenzin, Kya, and Bumi allow a hostile spirit to toss them into the Fog of Lost Souls, a place where human souls are forced to wander aimlessly for all eternity (one such victim is Zhao, who's been there for over seventy years).
Bumi and Kya succumb to the Fog's maddening effects, and Tenzin begins to as well until the spirit of Aang appears to him. Aang tells Tenzin that he is not Aang: he is Tenzin, and not a reflection of his father. Reinvigorated, Tenzin uses his Airbending to rescue Jinora, Kya and Bumi from the Fog. Meanwhile, Bolin is able to convince Eska to free him and Mako after he professes his love to her.
The battle between the Avatars spills over into the material world, and a boost from Raava gives Korra the strength to overpower the Dark Avatar. However, as they grapple, Vaatu exits Unalaq's body and forcibly rips Raava from Korra's. Unalaq then proceeds to destroy Raava, and with her, the spirits of all previous Avatars from Aang to Wan himself. Korra is no longer the Avatar.
Now the sole Avatar in existence, Unalaq grows into a monstrous Kaiju form, and departs the South Pole to begin his destruction of the world.
Was released a week early at midnight, the same day as the previous two episodes were aired. Can be watched on Nick's website here.
- And I Must Scream: The Fog of Lost Souls has kept some of its victims there for decades, maybe even centuries or millennia! Look at Zhao for example, he's been there for about seventy years.
- Anguished Declaration of Love: Bolin declares his love for Eska when she and Desna have them trapped in the spirit world, convincing her to release them so they can defeat Unalaq. Mako thinks it was an act, but Bolin clearly meant it on some level.
- Attack of the 50-Foot Whatever: The Dark Avatar's giant form.
- Be Yourself: When Tenzin is nearly consumed by his self-doubt in the Fog of Lost Souls, Aang appears before him, telling him that he's been trying too hard to live up to a legacy rather than being himself. Accepting this, Tenzin comes to terms with his shortcomings and overcomes the Fog, allowing him to save his family.
- The Cameo: Admiral Zhao makes an appearance, having been trapped in the Fog of Lost Souls. He's stuck ranting about killing the moon and capturing the Avatar.
- Continuity Nod: When Sokka tried the cactus juice he thought he saw a giant, possibly friendly, mushroom. What did his niece and nephews encounter in the Spirit World? A psychedelically glowing, talking, giant (compared to the usual real world size) mushroom. While the spirit wasn't exactly friendly, it didn't try to hurt them either.
- Darkest Hour: Raava's destruction, Unalaq's fusion with Vaatu and the effective deletion of Korra's link to the past Avatars may very well be the Darkest Hour of the entire franchise yet.
- Driven to Madness: Zhao's ultimate fate and punishment for murdering Tui. He's lost touch with all forms of reality by being imprisoned in the Fog, and keeps repeating the same spiel about being a moon-slayer. Zhao is so insane he even mistakes Tenzin for a grown-up Aang and tries to attack him.
- Evil Counterpart: Unalaq succeeds in turning himself into the Dark Avatar by fusing with Vaatu. Thankfully, he only has access to waterbending and the experience of a single human.
- Hope Spot: Many, but especially after Korra hears Raava speaking to her and returns to fighting Unalaq, telling him he can't win. She seems to get the better of him by locking arms with him with waterbending, but then Vaatu partially exits Unalaq's body, rips Raava out of Korra's, and then destroys her.
- Irony: Last we saw of Zhao, he smugly believed killing the moon spirit would make him a legend that would be remembered for ages. But his only legacy is his failure - Aang and Katara's own kids don't seem to recognize his name. He is doomed to wander the Fog, forever denying his defeat.
- No-Holds-Barred Beatdown: Once he's pulled Raava from Korra, Unalaq savagely attacks Raava with waterbending even though she's powerless to fight back, eventually destroying her.
- One-Winged Angel: When Unalaq destroys Raava, he transforms into a giant, humanoid version of Vaatu.
- Orifice Invasion: Unavaatu sends a Vaatu tentacle out of his mouth to forcibly pull Raava out of Korra through her mouth.
- Psychological Torment Zone: The Fog of Lost Souls in the spirit world is a massive spirit that erodes the wills of those within, causing them to be consumed by their nightmares. However, it becomes harmless if the victim can accept themselves for who they are.
- Scary Scorpions: A spider-scorpion takes Bumi, Kya and Tenzin into the Fog of Lost Souls.
- Thoroughly Mistaken Identity: Zhao genuinely thinks Tenzin is Aang all grown up, unaware that it's his son and Aang passed on almost two decades ago.
- Wham Episode: Unalaq and Vaatu fuse, becoming the Dark Avatar, and then separate Raava from Korra, destroying her and Korra's connection to all the previous Avatars. With Raava defeated, Vaatu and Unalaq go One-Winged Angel and prepare to bring on 10,000 years of darkness.
- What Happened to the Mouse?: Tenzin leaves at least a few dozen people still trapped in a daze after dispelling the fog. Since Tenzin had to drag his family away from the former fogged spot, it's uncertain if the people would have come to on their own or if they were still trapped in their own madness. Some of them were probably monsters like Zhao, but how many are innocents trapped in the Avatar version of hell?
- You Can't Thwart Stage One: Korra has Vaatu completely dead to rights and is seconds away from sealing him away for another ten thousand years. Outside, Mako and Bolin are managing to keep Unalaq away from the portal. Then Desna and Eska show up.