The Light & Shadow Trilogy — Moira Katson

It has been only two generations since Arthur Warden seized the throne of Heddred from the Conradines, and now the crown rests on the head of Garad, sickly and weak. Shadows gather: legacies of the centuries-long rivalries for power, old betrayals, the endless plots of the courtiers, and the murmur of rebellion in the southern provinces…

Catwin, plucked from her life at the edge of the Kingdom, is thrust abruptly into the world of the Court when she is chosen by the Duke of Voltur to be a Shadow -- spy, shield, and blade -- to his niece, the Lady Miriel DeVere. The Duke's ruthlessness is legendary, and he will stop at nothing to become the power behind the throne, using Miriel as a pawn to catch Garad's heart.

But the Duke's carefully-laid plans are only a piece of the intrigue of the court, and greater forces than Catwin can imagine are massed against her, determined to eliminate Miriel and impose a new order of their own. If Catwin and Miriel are to survive, they must learn quickly who to trust, and when to turn their skills against the very people who have trained them...