The Annihilation Score — Charles Stross

Dominique O'Brien -- her friends call her Mo -- lives a curious double life with her husband, Bob Howard. To the average civilian, they're boring middle-aged civil servants. But within the labyrinthine secret circles of Her Majesty's Government, they're operatives working for the nation's occult security service known as the Laundry, charged with defending Britain against dark supernatural forces threatening humanity.

Unfortunately, one of those supernatural threats has come between Mo and Bob. An antique violin, an Erich Zahn original, made of white human bone, was designed to produce music capable of slaughtering demons. Mo is the custodian of this unholy instrument. It invades her dreams and yearns for the blood of her colleagues -- and her husband. And despite Mo's proficiency as a world-class violinist, it cannot be controlled…