This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Charlie and the Great Glass Elevator
The Vermicious Knids are definitely Nightmare Fuel. They look like 4ft-tall furry eggs that stand on their pointy ends and have HORRIBLE staring eyes. They can twist into any shape and they eat humans. One of the most unnerving things about them is 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're the first things that Mr. Wonka is actually SCARED of. It could be difficult just turning the page, knowing that picture of the thing standing in the elevator will be there. Even worse with the Michael Foreman illustrations...that is, BEFORE Quentin Blake did them.
Especially how... innocuous they look 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 Knids do manage to attack and devour a bunch of unsuspecting visitors to the space hotel. What they exactly do to them is left to the imagination, but back on Earth the President and the White House staff hear what's going on over their radio...
Wonka mentions, while describing what the Knids are, that there was once a very friendly species that lived on the Moon. The Knids rendered them extinct.
Minusland — the place where those who overdo Wonka's Fountain of Youth elixir Wonka-Vite go. It's a misty area full of spirits — imagine this cavern with negative ghosts floating sideways...
Grandma Georgina being aged to several hundred years old and her resulting withered appearance is enough to severely freak out the rest of the Buckets and even Mr. Wonka. Bear in mind that this happens just hours after the group had been threatened with being eaten by Vermcious Knids.
Turned Up to Eleven in the first UK version of the book, which uses a highly detailed and realistic illustration style, as opposed to the more cartoonish and abstract styles employed in other versions of the book. And the illustrator does not hold back in depicting just how decrepit she ends up looking.