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Nightmare Fuel: Charlie and the Great Glass Elevator
The Vermicious Knids were definitely Nightmare Fuel. They looked like 4ft-tall furry eggs that stood on their pointy ends and had HORRIBLE staring eyes. They could twist into any shape and ate humans. One of the most unnerving things about them was that it's suggested that they don't eat people so much as absorb them - they don't have mouths, but "they have other things to bite with". They were the first things that Mr. Wonka was actually SCARED about. It could be difficult just turning the page, knowing that picture of the thing standing in the elevator would be there. Even worse with the Michael Foreman illustrations... that is, BEFORE Quentin Blake did them.
Especially how... innocuous they looked initially. With both artists, the first illustration is just this furry egg-thing with eyes, no mouth, no nose, no huge fangs or fire-breath or enormous wings- not, in essence, something to be feared. And then we find out that they are.
At one point, five Knids twist their bodies into the word "SCRAM". The heroes do. And then Charlie asks why they'd tell you to scram if they want to eat you... and Wonka explains that they like to prolong the chase before they attack. These things are sadists on top of everything else.
The bit going down with the 'Minuses' to rescue the grandmother who'd taken too many of the de-aging pills and had been reduced to -3 years old. Imagine this cavern with negative ghosts floating sideways...
Oh, and the cavern is also populated by Gnoolies, who are invisible, inaudible creatures that, if they bite you, you become one of them. And there's no way to tell where they are or if they're coming until they bite you...