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troperville

The Body Snatcher possession of Vachir by Heian Chao. Not only does the reader get a front-row seat to it happening, from Vachir's perspective, but comparisons are drawn to the way the Shadowen take over people in the The Heritage of Shannara, and at one point the reader gets to see Chao literally draw out and torture Vachir's soul. The fact this all leads into his body then going on a mad murder spree, with Vachir trapped inside unable to stop it, only to scream and beg for a death he can't have, is just the icing on the cake.
The murdered goat. While other deaths which follow this are bloody and graphic, this is the first one to be described in explicit detail, and both it and the reactions to it are where the story first really hits the Darker and Edgier line.
Monkey's Room Full of Crazy: "I like dead bananas." That the trope appears at all is disturbing enough, but the nature of it (numerous drawings of Po, Shifu, and the Five being done away with in messy ways, obsessive rants against Tai Lung, "DIE DIE DIE" being written everywhere) and whose room it is are what push it over the edge. It even scares Po and Jia.
Qinghe, in the Nothing Is Scarier sense. A totally empty, abandoned town, where everyone has fled for an unknown reason into the desert, leaving behind toys, clothes, food, weapons, everything they would need to survive...eye-like windows and open doors everywhere, no one manning the ferry, the fields filled with only the whisper of the wind...and every grave in the town cemetery open, the bodies all taken with them.
The Changs� basement and what Zhuang finds there: the two elephants left suspended from grappling hooks from the ceiling, slowly and agonizingly bleeding to death, with Chao living off of and growing powerful from their fear, horror, and pain, while being forced to look at their dead son, until he finally allowed them to die. Oh, and Chang had suffered Eye Scream.
Chao corrupting the Sacred Pool. The fact the author explicitly identified the scene as a Shout Out to Chernabog's "Night on Bald Mountain" sequence should be enough all by itself, and the idea of the pool becoming a thing of evil and horror would be disturbing too, but there's also the fact that the profaned waters actually kill all life around them as they flow down the mountainside. That this is all just the precursor to the whole Valley becoming People Puppets only makes it worse.
Shifu�s premonition. In general the scene is practically a re-enactment in vision form of Chao corrupting the Sacred Pool and a Foreshadowing of the upcoming People Puppets. Add in the sense that doom is coming for Tai Lung, and the whole Valley...
The trial, with everyone as People Puppets. It's bad enough it's a Kangaroo Court out for blood, but when a bit of aid from Oogway's staff lets Shifu see what is really going on...well, wouldn't you be rather chilled and terrified to see everyone around you as a shadow-cloaked, red-eyed, demonic figure?
Chao's One-Winged Angel moment. The description has definite shades of the Firebird.
Jia aging. As if the concept itself wasn't bad enough, the description has a number of things in common with the death of Donovan and it was fairly clear if he hadn't been stopped, Chao would have aged her to her death.
The gigantic demon which the yaoguai join to form. It's storyboarded in Art of Kung Fu Panda and looks rather terrifying there. Its appearance in this story lives up to the imagery.
Chao's death. Not just Impaled with Extreme Prejudice, but roasted by holy fire, with a melt-and-dissolve worthy of Raiders of the Lost Ark, and then he explodes.