Kishin Kakka No Migawari Hanayome

Tsukiko is a young woman who has been a sickly and frail child, and is treated as a good for nothing. She becomes the replacement for her twin sister who was going to marry His Excellency, Kishi, a man who rose to become a Lieutenant General of the army at a young age. Due to his ruthlessness, he gained the nickname of Demon God. She met him once before, and his kindness became the one thing that supported her. But instead of that kindness, he bluntly says, if you can fulfill your duty and give birth to a boy that will become my successor, I will divorce you without making a fuss.