The Assassin and the Underworld (Throne of Glass #0.4)

When the King of the Assassins gives Celaena Sardothien a special assignment that will help fight slavery in the kingdom, she jumps at the chance to strike a blow against an evil practice. The misson is a dark and deadly affair which takes Celaena from the rooftops of the city to the bottom of the sewer--and she doesn't like what she finds there.