King's Beast, Vol. 15 (manga)

On the eve of war, a letter from a potential ally arrives at the imperial palace, foretelling Prince Kougai’s coming death in battle. The letter is persuasive enough that Prince Tenyou and the emperor begin to make plans to circumvent their brother’s dark fate. But according to the letter, the only way for Prince Kougai to live is for someone else to die in his place!