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Nightmare Fuel: A Wrinkle in Time
The Black Thing
IT, a giant disgusting brain that enslaves your circadian rhythms if you get close to IT. It's the Hive Mind for the entire planet of Camazotz, swallowing everyone else's minds.
An actual nightmare: all roads lead 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. Then he takes control of Charles Wallace.