Libertine Prince And Captive Beast summary:

Alain, the second prince of the Kingdom of Cors, has faced a dire problem. His mother, the king’s concubine, decided to poison the crown prince to make Alain the next king. However, Alain doesn’t wish to harm his brothers and join the 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?!