What was IT and what could IT do?

In Madeleine L'Engle's A Wrinkle in Time, IT represents evil and darkness. IT is a disembodied brain that lives on in Camazotz. When Meg first encounters IT, the narrator explains:

A disembodied brain. An oversized brain, just enough larger than normal to be completely revolting and terrifying. A living brain. A brain that pulsed and quivered, that seized and commanded. No wonder the brain was called IT. IT was the most horrible, the most repellent thing she had ever seen, far more nauseating than anything she had ever imagined with her conscious mind, or that had ever tormented her in her most terrible nightmares. (Ch. 9)

IT uses pulses to communicate messages, and once a person is entranced by the rhythm, they are pulled in and more easily convinced by the arguments IT makes. IT creates a reality for the people of Camazotz that IT completely controls. People live in identical houses, wear identical clothes, and behave in identical ways. Charles Wallace initially wonders if the people of Camazotz are robots rather than humans.
Charles Wallace is taken in by IT, causing distress for Meg. She's so angry at the IT version of her brother that she tries to hurt him. In chapter 9, entitled IT, Meg and Charles Wallace have located Mr. Murray, but Meg can't get to him, and in her frustration, she dives for her brother.