Maskerade:

The Opera House, Ankh-Morpork...A huge, rambling building, where innocent 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 (but nevertheless a good evenin's entertainment with murders you can really hum...).