Maskerade (Discworld Novel 18)

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. But Granny Weatherwax 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 evening’s entertainment with murders you can really hum.)