Elizabeth Vaughan: Warprize [2007] paperback

Xylara is a Daughter of Xy, brought up to do her duty to her country, her king and her people. Her father, the Warrior King Xyron, is dead and her incompetent half-brother Xymund is on the throne; she has chosen to serve her people as a healer, until she is needed to make a marriage of alliance. But the once-great kingdom of Xy is threatened by the Warlord and his barbarian Firelanders, and her brother Xymund is forced to surrender. The terms are fair: the kingdom will remain under Xymund's control, and the taxes and tithes are reasonable. All prisoners and wounded are to be exchanged unharmed. There is just one thing more: to ensure a true peace - no pillaging, no looting, no rapine - the Warlord has claimed tribute. He claims Xylara, Daughter of Xy, to be his Warprize ...