This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / A Wrinkle in Time
The Black Thing - the grandaddy Eldritch Abomination, a kind of Outer God composed of pure, black evil. And it's always hungry.
IT, a giant disembodied brain that enslaves or outright absorbs your mind if you get too close to IT. IT is the literalEldritch Abomination and Hive Mind that runs the entire planet of Camazotz, subsuming any dissenting minds within ITself and cowing the rest into fearful, blind obedience.
And it's rammed home exactly what kind of eldritch horror the kids are dealing with when Charlesnote All references to Charles in this section refer to Charles under ITs control. Normally, Charles was a loving little brother toward Meg. lets slip how IT thinks of ITself:
Charles: IT sometimes calls ITself the Happiest Sadist.
An actual nightmare: all roads on Camazotz lead ultimately to IT.
At one point, the kids are accidentally transported to a two-dimensional planet for a few seconds. It nearly kills them.
ITs servant, The Man With Red Eyes, who talks about the potential enslavement of the universe in the politest terms possible and doesn't open his mouth when he talks. He lets the mask slip when he actually laughs at one point, a laugh that would not be out of place from a Lovecraftian Outer God. Then he channels IT to take control of Charles Wallace...
The rather casual, banal way which Charles explains how Camazotz deals with illness - even minor ailments such as the common cold.
Charles: We let no-one suffer. It is so much kinder to simply annihilate anyone who is ill. Nobody has weeks and weeks of runny noses and sore throats. Rather then endure such discomfort, they are simply put to sleep.
Calvin: You mean they're [literally] put to sleep while they have a cold, or that they're murdered?