Emperor Mage by Tamora Pierce

Daine reluctantly sails to Carthak as part of a peace treaty delegation from Tortall. She is to examine the Emperor Orzone's sickly birds. While healing the captive birds, Daine discovers a new and terrifying dimension of her wild magic: She can bring dead creatures back to life. What is she to do with such immense power? Finding out the answer nearly costs Daine her life, but slowly she uncovers the darkness that is coursing through Carthak and the evil that threatens the fate of the peace treaty. As the gods plan vengeance on Carthak for the terrible deeds the empire has committed, Daine must learn to wield her newfound power to thwart their divine retaliation and secure peace for all of Tortall.