Maskerade : (Discworld Novel 18)

The Opera House is a rambling building, where young sopranos are lured to their destiny by a strangely-familiar evil mastermind in a hideously-deformed evening dress... At least, he hopes so. But Granny Weatherwax, Discworld's most famous witch, is in the audience. And she doesn't hold with that sort of thing. So there's going to be trouble.