Return of the Thief by Megan Whalen Turner

Neither accepted nor beloved, Eugenides is the uneasy linchpin of a truce on the Lesser Peninsula, where he has risen to be high king of Attolia, Eddis, and Sounis. As the treacherous Baron Erondites schemes anew and a prophecy appears to foretell the death of the king, the ruthless Mede empire prepares to strike.