Emperor's Hostages

Wren is a shape shifter whose magic is flawed; every promise he makes takes unexpected turns. One failed promise caused his enslavement to barbarian royalty. The tale begins a thousand years later, when an emperor, who collects hostages to ensure the obedience of conquered kings, conquers the barbarian kingdom where Wren serves. Snatching at an opportunity to escape bondage, Wren offers himself as hostage for his king, promising to win their freedom.
However Wren's first act, upon meeting the emperor, is to insult him. Events worsen when Wren befriends a hostage prince, involves two lovers in a struggle for the throne, helps to raise an army for them, dodges the thrusts of an oracle who can't forgive Wren for impersonating him, and faces the dilemma of saving a princess's life or dying by his own magic.