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Nightmare Fuel: A Hat Full of Sky
The Hiver hides in people, but their brains die. They always die.
As it went past, it made a noise like a swarm of flies.
A guy was turned into a frog. As a human being has far more mass than that of your average frog, the rest of him turned into a slushing pink balloon. He got better.
It's often mentioned in previous books that Granny Weatherwax very much prefers to make people think they've been turned into frogs than actually doing it, played for laughs with the line "why go to all that trouble". Now we know what that trouble is...