Maskerade : a Discworld novel

The show must go on, as murder, music and mayhem run riot in the night...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...)