Nightmare Fuel / Hero: 108

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• The leeches in the episode "Leech King" when they're masquerading as the members of Big Green. They have unnaturally green eyes and weird black goo oozing from their eyes, not to mention they also stretch and bend in horrific ways and speak in a rather disconcerting manner. And in one of the scenes, before the leeches begin replacing ApeTrully and First Squad, the episode provides us with... this. Just try to imagine a couple of big worm-like creatures crawling out of your eye sockets...
  Leech!Mighty Ray: We have been waiting for you, Mystique Sonia.
  Leech!Jumpy: Your... turn...
  Mystique Sonia: My turn for what? Why are you guys acting so creepy?
• In "Pitched Battle of the Air Force Part 2" we get the Twelve Eyed Striped Dumbat, a dangerous fusion of Big Green's enemies created by Mighty Ray toying around with Woo's invention. First off, this thing is enormous, and very terrifying with its might. The monster takes Jumpy, Sonia and Mighty Ray hostage, who then break free from its grasp and start assaulting the heads, seemingly having the upper hand. But then, HighRoller grabs them with Bearstomp's arm and pulls them into his mouth, swallowing them in the process. Not only is HighRoller satisfied right after (even commenting on their taste), there's also the fact that two of the victims are humans. He literally commits cannibalism right then and there, without a single second thought. Not helping matters is that the original storyboard has an unused scene that shows what would've been Lin Chung's reaction: horror.
  Lin Chung (in the storyboard): He swallowed my teammates!
• Twin Masters. Although the show is typically lighthearted, Twin Masters mean business and have a genuinely intimidating appearance of a gargantuan, half-blue and half-red, vaguely humanoid entity with a skull sitting on top of its incredibly wide torso and twelve hands (two big ones and one on each finger), not to mention their voice is as equally intimidating and their motive being plunging Hidden Kingdom into everlasting chaos.
  • They go One-Winged Angel in the Series Finale, turning into something that resembles a giant snake-bodied Eldritch Abomination and being bent on destroying all that's alive. They become so powerful that even the upgraded, mobile Big Green base can't do anything to them, instead laying waste to it and nearly killing everyone inside. The only reason anyone even survives is because of Mystique Sonia's quick thinking. Twin Masters also drop a really big Wham Line when letting HighRoller and his goonies know that they will perish too despite their allegiance, adding to their already terrifying status.
    Twin Masters: Hidden Kingdom is now ours for the taking! Once we ingest this pathetic world, there will be nothing left alive.
    HighRoller: You mean, no heroes left alive... right, Twin Masters?
    (Twin Masters swat HighRoller)
    Twin Masters: Nothing. Left. Alive. Your services are no longer required...
• The titular Swamp Hippos that appear in the episode of the same name. On the surface, they are shaped like hippos... but when they start attacking, their entire body opens up and reveals itself as one giant mouth. The words "absolutely freaky" don't even begin to cover it.
• In "Penguin Castle", the Penguin King's method of grabbing a shard of the aurora involves his skeleton launching out of his body and flying like a rocket into the air, which is very unsettling.