The ending of F.C. Yee's The Shadow of Kyoshi has the darkest ending in the Avatar: The Last Airbender franchise. The follow-up to 2019's The Rise of Kyoshi, the climactic battle of The Shadow of Kyoshi sees its title character and her ally Rangi attempting to put a stop to the unhinged Yun, who more than holds his own against both of them. As it is, Yun himself is arguably the most sympathetic villain the Avatar franchise has ever produced.
Yun had been mistakenly identified as the new Avatar in The Rise of Kyoshi, only for him to be betrayed by his mentor Jianzhu, with Kyoshi being revealed as the actual Avatar by Father Glowworm. In the pages of The Shadow of Kyoshi, Yun finds himself in the Spirit World and is offered a deal to join forces with Father Glowworm. However, Yun recognizes this for the Faustian bargain that it is, instead consuming Father Glowworm's essence and emerging from the Spirit World far more powerful and ready to take his vengeance on the world.
In the finale battle, Yun, clearly prepared to shed the blood of anyone standing in his way, coldly stabs Rangi in the back. As Kyoshi holds the badly wounded Rangi in her arms, she realizes that Yun cannot be reasoned with or pulled back from the abyss. After apologizing to the approaching Yun that she "stole your Avatarhood", Kyoshi puts her hand to his body and icebends his heart and lungs frozen, killing him.
Both Rangi and Kyoshi mourn for Yun as the former recovers from her near-fatal stab wound, and this stands as a major contrast to the conclusions of Avatar: The Last Airbender and The Legend of Korra. Even in pitched battle against the ruthless Fire Lord Ozai, Aang could not bring himself to take a life, instead removing Ozai's Firebending from him with the Energybending he had been empowered with by a Lion Turtle. Korra also managed to end Kuvira's attack on Republic City without killing her in the finale of The Legend of Korra, while the show concluded with Korra and Asami Sato's trip into the Spirit World, itself very heavily hinted at being a romance between the two in the final shot.
Though Kyoshi managed to put a stop to the carnage Yun was ready to unleash, The Shadow of Kyoshi ends on a far more somber note than the triumphs of Aang and Korra. The epilogue also sees a messenger sent by Kyoshi with a threat to Fire Lord Zoryu in light of his Machiavellian plan at maintaining order in the Fire Nation. Despite the inherent difficulties of being the Avatar, the final pages of the book show just how seriously Kyoshi takes her duties, and that she is entirely willing to pull rank on the leaders of the Four Nations if she must.
The Avatar franchise has been widely praised for its accessibility to children and adults alike, but The Shadow of Kyoshi takes it into far darker territory than it's ever ventured into, especially in its ending. Yun's demise is presented as a genuine tragedy, while the way in which Kyoshi relays her message to Zoryu shows that she has no problem getting her hands dirty if the need arises. Avatar has incorporated dark storytelling and emotional stakes plenty of times before, but never has it put forth an ending as dark as that of The Shadow of Kyoshi.