Houtou Ouji to Toraware no Kemono

Alan, the second prince of Corus, asks Sauro, a warrior of Matara who is being held in a dungeon, to help him escape in order to stop the civil war that was planned by his mother, the mistress of the king, to usurp the throne. Saul asks Alan for a "vow" as a condition of his cooperation, but that "v