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.
And it's rammed home exactly what the kids are dealing with when Charles lets slip how IT thinks of itself:
Charles: IT sometimes calls ITself the Happiest Sadist.
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.
The rather casual, banal way which Charles (under control of IT) explains how Camazotz deals with illness.
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?
Charles: "Murder" is a most primitive word...
Tessering: what does nothingness actually feel like?