The Daughter of Crows

Set a thief to catch a thief. Set a monster to punish monsters.
The Academy of Kindness exists to create agents of retribution, cast in the image of the Furies - The Kindly Ones - against whom even the gods hesitate to stand.
Each year one hundred girls are sold to the Academy. Ten years later only three emerge.
The Academy's halls run with blood. The few who survive its decade-long nightmare have been forged on the sands of the Wound Garden. They have learned ancient secrets amid the necrotic fumes of the Bone Garden. They leave its gates as avatars of vengeance, bound to uphold the oldest of laws.
Only the most desperate would sell their child to the Kindnesses. But Rue ... she sold herself. And now, a lifetime later, a long and bloody lifetime later, just as she has discovered peace, war has been brought to an old woman's doorstep.
That was a mistake.

Mark Lawrence