Houtou Ouji to Toraware no Kemono

Alain, the second prince of the Kingdom of Cors, faces a dire problem. His mother, the king's mistress, decided to poison the crown prince to make her own son the next king. However, Alain doesn't wish to harm his brothers and fight for the throne, so he asks Saul, a Matara warrior trapped in the castle's dungeon, for help in order to flee from Cors. To receive Saul's cooperation, Alain must make a special vow with him and confirm it... with his body?! (Source: MangaDex)