A dark ritual goes awry and now the great Spirit of the Dark walks the earth once more. His power is great, and he seeks only one thing: Vengeance upon the one who imprisoned him centuries ago.
Aang's eyes and arrow tattoos were glowing with a brilliant light as he seized the earth beneath them and forced it up violently, sending Koh flying. The smallest flick of his fingers, and an enormous stone pillar shot out of the ceiling as a downwards angle, catching Koh about the midsection and slamming him into the ground with enough force to send cracks cascading through the surrounding stonework.
Aang gestured upwards, and a geyser broke out of the ground beneath Koh, pushing him up into the air once more. The Avatar pulled the water back towards himself, swirling it about, and then sent it back at Koh, freezing it into an icy lance.
Koh twisted in midair, inhaled sharply, and then expelled from his maw a beam of pure darkness. It split the icy lance in two, sending shards of frozen water clattering to the stones.
Koh landed roughly, but quickly caught his feet again and surged forward. This was what he had been waiting for. If he had simply killed Aang, his brother's spirit could move on to somebody else. But by forcing him first into the Avatar State, he could strike directly at his brother, and kill him.
The ground beneath him surged upwards, trying to surround and crush him, but Koh projected a web of darkness around himself, and the rock disintegrated as he pushed through.
Aang threw both hands forward, and hurricane-force winds burst forth, buffeting Koh backwards. Koh pushed against the wind, slowly but surely pressing his way through the gusts. He fired another dark beam, which Aang easily dodged. But then the beam looped around, returning to connect with itself, trapping Aang inside.
Koh pulled on the tether, pulling Aang towards him. Aang severed the cord with a slice of compressed air, then attacked with two enormous gouts of flame.
Moving with inhuman speed, Koh leapt over them, landed and rolled to avoid another assault. He dashed forward, wrapping around Aang, getting behind him. Aang whirled to face him-
-And found himself inches away from Katara's terrified face.
The tortured expression appeared on his face only for an instant. But for Koh, that was enough.
"You're mine!" he crowed triumphantly, lunging forward. Katara's face vanished, replaced once more by that dark maw. He pried Aang's face off with a sound of supreme satisfaction.
He was therefore very surprised when a bust of light flared out from where Aang's face had been, forcing him away.
In the aftermath of Shinbei's fall, the other spirits began to grow tired. When the world was new, they were children. When the world was shaped, they were adults. Now the world was balanced and complete, and they were old.
Spirits do not die naturally. Sometimes they fight, and they are even killed, but old age will never take one. But the eight spirits who had formed and ruled the world did the next best thing.
One by one, they encased themselves in mortal forms, and they slept within them. Their power was still there, and at a moment's notice could be released, but in these forms they could at last rest.
Tui and La, who had always been the closest of the Spirits, took the form of twin Koi fish, and took their place in the Spirit Oasis of the Northern Water Tribe. Tui took the form of a white fish with a black mark on her forehead, and La the form of a black fish with a white mark on her head. Around and around the pond they circled, forming the symbol of the Push and Pull that they represented.
Gan took the form of an enormous Badgermole, and stalked the earth beneath a great mountain. Eventually, a pair of lovers from warring families found him about his travels, and by imitating his motions refined and mastered their Earthbending abilities, bringing them to never-before-seen levels.
Basha took the form of a great Skybison and roamed the air currents, controlling and bending the wind to her purposes. Eventually the nomadic monks that had been her favorites imitated her, incorporating her movements into their bending style.
Ilza took the form of a great eaglelizard, proud and majestic and as fierce as the Sun he ruled over.
Ashin took the form of a powerful Dragon, a ferocious beast with a fire burning in his belly. His shape became the most revered image in the entire Fire Nation, and wherever he went he had his appetites sated.
The form that Tysha chose for herself surprised everybody. All who knew her would have expected her to take the form of a winged lemur, or a lizardchimp, some playful trickster animal. Instead she smiled wearily, said that she was tired of jokes, and became an enormous tree. A great swamp grew up around her, and through it she was connected to each and every living being across the earth. Eventually, a small tribe of Waterbenders moved in and lived beneath her branches. And although she claimed to be sick of jokes, from time to time she still indulged in her old ways, summoning up air currents and visions to fool and trick passerbys.
Wan Shi Tong took the form of a great owl, wisest of beasts, but would not rest as his siblings did. He claimed that he still had work to do, and closed himself off in his library, sending his spirit foxes out into the world to gather information to add to his great Record. As always, he did nothing, and recorded everything.
And so the Great Spirits left us, placing the fate of the world in the hands of Man. And foremost among those was the Avatar.
Koh fell to the dirt, reeling back from the luminous vision before him. Brilliant light was flowing from where Aang's face had been, looping out and around him, forming a golden corona. This was not the pure white light of the Avatar State. It was a soft, golden light, which Koh knew all too well.
Aang's mouth opened, and he spoke, but the voice was not Aang's. It was a powerful, commanding voice, with harmonics inflicted with every syllable.
"Foolish, foolish Facestealer!" the presence inhabiting Aang roared. "Did you truly believe me so easily defeated? That taking this young boy's face would do anything but unseal my power? Surely even you knew better than that."
Koh hissed furiously and fired a beam of darkness at the hovering figure, but Aang raised one hand and deflected it effortlessly off to the side.
"Is this what you wanted? A duel between us? You were never my equal, even before the Creator empowered me to uphold its will. The darkness always retreats before the light."
Koh shot off a net of his dark stuff, but it vanished as it approached the golden glow surrounding Aang's body.
"Enough of this," the Great Spirit of Light said derisively. "You were warned of the consequences of return when you were first banished from this plane. That you ignored them and then did everything short of actively court judgment only reveals the depth of your foolishness. This time, I will destroy you completely, as I should have done centuries ago."
"Mercy, Brother, please!" Koh cried frantically. "I'll return both their faces, I'll return to the darkness, never to be seen again, only let me live, I beg you!"
"Return them? I think not. I will take them back, along with every other face you have stolen!"
Aang extended one hand, and a beam of radiant golden light shot from it, impaling Koh right through his midsection. Koh began writhing and screaming as the beam melted through his exoskeleton.
"Brother, please!" Koh shouted in agony, "You need me! You need me to maintain the Balance! I am your dark shadow, without me you are nothing! Please, brother, spare me!"
"The Balance? Do not mock me, Ebesh Fleshrender. I know better than any what is needed for the Balance, and it is not you! When last you betrayed us, the Creator spoke to me. It told me that, should you go against Its will again, I was to destroy you completely, and leave nothing behind. Another will rise to take your place."
The beam moved along Koh's body, cutting through the shell and splitting him open. Koh was beyond speaking at this point, and could do little more than scream incoherently as Arventi's light seared through him.
"You narrowly avoided this fate long ago, when the Creator took pity on you. But It is not here now, and I am the one who is passing judgment on you. For your crimes, I sentence you to oblivion."
Koh's insectile body split open, revealing a gruesome sight. All throughout his body, in lieu of internal organs, he had thousands upon thousands of face, revolving, turning, milling about and flowing.
As if sensing a chance at freedom, the faces poured forth from him. There were far, far more in there than could possibly have fit, but they kept coming anyways, pushing out of his split body and grasping at the sunlight. All but two bore the same empty expression.
When at last they had all emerged from their prison, the beam of light intensified, incinerating what remained of Koh's body. Aang turned to face the sea of faces, which were surrounding him, staring expectantly.
He bowed low, in a huble and respectful manner.
"I sincerely apologize for my brother's behavior," Arventi said through Aang. "If I had been able to, I would have freed you at the earliest possible opportunity."
He straightened, and addressed them one final time.
"Go now, and return to your bodies."
It was a moment before the first face tentatively floated away. The second followed shortly afterwards. Then another, and another, until there was a flowing whirlpool of faces, taking off and floating away to be reunited with their bodies. And although none of them spoke, each of them said thank you nonetheless.
Faces streaked across the sky, seeming to the people below to be as shooting stars. Everywhere a face had been taken, they went. They slipped quietly into graveyards, under tombs, and into the wilderness, to the bodies that had never been found.
A few miles away, Katara opened her eyes once more with a wrenching sob.
"Katara!" Sokka cried out happily, dashing to her side. She threw herself into his arms, weeping hysterically.
"Oh gods, Sokka, it was so awful...it was like there was this little dark corridor, and you kept going around and around and around, and you knew that you were never getting out and nobody was coming to save you and..."
"It's okay now," Sokka said, holding her tightly as she babbled on, "You're safe now." Mentally he thanked Aang for whatever he had done as he held her until she stopped crying. After a few minutes, even normally-aloof Toph joined in, lending what comfort she was able.
And then, when every one of the faces had returned to its rightful owner, Aang's face floated onto his head, and the light shut off. Aang collapsed on the floor, asleep.