Discworld book where the auditors hear about the phrase "it's not over till the fat lady sings" and hire someone to kidnap all the fat ladies in the world to find the One who will end the world when she sings.
Lady Sybil Vimes is one of the ladies, so Sam Vimes is on the warpath until he can find her, while the watch desperately try to keep all infrastructure from falling apart without all the fat ladies who keep it together on a daily basis
It ends when Sybil leads a hoard of fat ladies into battle, which ends up being so glorious an unrelated time traveler who witnesses it goes back to his native time starts the myth of the Valkyries











