From the inside flap:
In a world where people born with an extreme skill, called a Grace, are feared and exploited, Katsa carries the burden of a skill even she despises: the Grace of killing. She lives under the command of her uncle Randa, King of the Middluns, and is expected to execute his dirty work, punishing and torturing anyone who displeases him.
When she first meets Prince Po, who is Graced with combat skill, Katsa has no hint of how her life is about to change.
She never expects to become Po's friend.
She never expects to learn a new truth about her own Grace- or about a terrible secret that lies hidden far away... a secret that could destroy all seven kingdoms with words alone.