I recommend the book " Bloodthirsty Prince's Cold Concubine ". The male protagonist was a prince from another world. He was cold and bloodthirsty, but he loved the female protagonist unconditionally. The female lead was the mercenary king of the 21st century, and she also loved the male lead wholeheartedly. The two of them joined forces to fight against the enemy and create their own legend together. I hope you like my recommendation.