Moto Dorei no Migawari Hime wa, Reiketsu Ouji no Hanayome ni Natte Dekiaisareru

Prince Theodosius is rumored to have killed other princes as a political ploy in order to win the position of crown prince. Princess Beatrice is to marry such a man. However, her true identity is Lillian, a slave with the same eyes and hair color as Beatrice, who did not want to marry Theodosius.... A fantasy love story with a cold-blooded prince that begins with a substitution!