Kishin Kakka no Migawari Hanayome

Tsukiko is a young lady who was considered "a good for nothing" since she was a sickly and weak child. She takes over for her twin sister, who was engaged to His Excellency Kishi, an army lieutenant general who became a Lieutenant General at an early age. He became known as the "Demon God" because of his brutality. She had already met him previously, and his generosity had been her linchpin. Instead, he states frankly, "I will divorce you without a fuss if you can accomplish your responsibility and give birth to a boy that will become my successor."