Kuruk's attempts to reach out to Kyoshi, which while only occurring once (in a relatively stable fashion) in Rise, now apparently cannot happen without immense pain, resulting in Kyoshi being pitched off Jinpa's Bison into the freezing depths.
Said painful vision occurs above an island sacred to the Air Nomads, and was submerged by Kuruk in the Avatar State. Kuruk sank an island, more or less by accident.
“Kyoshi, an Avatar communing with their past selves is supposed to be a hallowed experience, not a life-threatening seizure!”
Kuruk's illness from fighting the dark spirits. He is wracked with lifelong (not that he lives long) pain, at one point coughs up blood, has chills, and becomes a wine addict. Is it any wonder he died at 33?
YUN.Yangchen protect us, Yun. He has apparently gone completely mad, ends up killing, then absorbing Father Glowworm, then when he arrives outside of the Spirit World, ends up beginning a bloody path of revenge, such as holding up the Fire Nation party Kyoshi and Rangi were attending, bending away the portraits of the Fire Avatars defyling a centrary old practice and erasing Fire Nation History, turing the Earth pigments into a bomb that wounds and kills several guards and Chancellor Dairin, and somehow managing to bend earth like water to the point where Kyoshi herself assumes he is genuinely an Avatar (if only partly) now. By the end of the book Kyoshi realizes that his quest for revenge will never be satisfied because he considers everyone in the world to be responsible for "lying" to him. At this point he can be considered a Omnicidal Maniac.
His descent into villainy begins with his being cruelly refused a drink of water by the owner of a tea house (after he'd nearly died from his experiences in the Spirit World!) leading him to start furiously ranting at the customers for their ingratitude after how hard he's worked to be the Avatar for them. A terrified mine worker tries to defuse the situation by offering him her water, but it's too late; Yun creates a blade from the ceramic pots around the building and murders everyone inside. And he smiles at them. Then proceeds to kill everyone else in the village who wasn't fast enough to flee and mocks the corpse of one of his victims, while bathing himself with buckets of the well's water again and again until he finally washed the blood off himself, making jokes through it all.
The worst part about Yun is how to some extent he is still the kid Kyoshi knew, to the point where Kuruk himself has to inform her that this is all his own doing.
Further, the way Kyoshi kills Yun, by freezing his heart and lungs solid.
Kyoshi's method of getting Chaejin to confess about his clan's affiliation with Yun is to confront him with his mother on the edge of a cliff, while Kyoshi and Chaejin are on the back of Yingyong the airbison. When the pair deny being involved with Yun, Kyoshi causes the cliff to nearly crumble beneath Huazo and threatens to drop Chaejin into the sea below, coming very close to killing them.
Fire Lord Zoryu reveals his true colors. A cunning, cold and manipulating mastermind who appeared before Kyoshi as a lost child is now taking advantage of Yun's attack to move against his political enemies, planning to kill the whole Saowon clan to make an example of them. The contrast between the humble and insecure boy and the Ozai-esque cruelty he shows later is quite chilling.
