This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Kung Fu Panda 3
The jade zombies, aka "jombies." They're basically shells created by Kai of former Masters that he uses as his eyes and ears, and they are Uncanny Valley personified by looking like not-quite-right jade statues of fallen fighters and moving in a jerky, unnatural way.
Kai himself. While he has some comical traits, he's terrifying. At some points, he comes off as something out of a slasher movie. The fact that, unlike Tai Lung's daddy issues with Shifu and Lord Shen's Parental Abandonment backstory and Pet the Dog moments towards the Soothsayer, Kai is simply a power-hungry devil whose only redeemable quality, his friendship with Oogway, he himself abandoned.
To emphasize how twisted his sense of morality is, Kai tirelessly carries Oogway for days to find someone who can heal him after he is injured. When he finally finds the pandas and they restore his best friend to health, how does he respond? By immediately turning on them and trying to steal their chi. 500 years later, he still has no idea why Oogway took issue with this.
What Kai did to achieve the level of power he had; defeating and stealing the chi of deceased Kung Fu masters in the Spirit Realm. All of these victims had lived their lives and passed on...only to run afoul with a demonic beast like Kai in the afterlife.
The buildup to Kai's arrival in the mortal realm features some pretty sinister music as he travels through the heavens before doing so.
Kai arrives at the Jade Palace, surrounded by a growing, green storm of bad juju. When he finally appears personally, he slowly lumbers the Palace warriors, glaring intensely, swinging his weapons and scraping them on the ground. He does lose a bit of the tension once he speaks, and cracks a joke about Oogway's statue, but it's one hell of an entrance.
Towards the end of the film, the Panda Village stops and looks up as they hear distant scraping sounds. As the sky darkens and begins to turn green, Tigress stops what she's doing and looks resigned to their fate, and knows what the scraping means: Kai has arrived.
The Wuxi Finger Hold itself. Knowing from this movie that it's an instant, certain one-way trip to the Spirit Realm, it's no wonder characters were terrified of it.
When it's time for Po, to take on Kai, Po does fairly well at first, but when Kai reabsorbs the chi, causing Po to lose his balance, he managed to land a hit on Po that sends him through a gate, destroying it. As soon as he gets to his feet, the first thing he can say to those nearby is a terrified "RUN!"
Kai's final demise. Kai was a spirit warrior and therefore in someway undead, not saying he didn't have it coming, but when Po overloaded him with chi...he didn't just kill Kai...he obliterated him from existence itself.
Kai's death seems to be more brutal than Tai Lung and Shen's.