From Renta!:

After many years, the Great War between the human - elf allied army and the demon horde ends when the allied army's commander - in - chief, Ares, falls ill and is forced to withdraw from the front lines. The victorious demon king, Johann, becomes the ruler of the world, and surprisingly, maintains lasting peace across the land. One day, Johann meets the priest Eugene, who is gifted with a special kind of magic power. Despite their racial difference, Eugene earnestly proclaims his admiration for Johann. Intrigued, and hoping to taste that special magic, the king lures the priest into his bedchambers and practically forces himself on him, claiming this would be an honor for Eugene. When Eugene calls out in surprise, a powerful magical barrier activates, sending Johann flying across the room...! This is a romantic comedy starting from a fateful encounter between a cocky demon king and a priest whose past intrigues him to no end!