Koushaku Reijou no Rouraku Mission: Maou to no Seiryaku Kekkon ga, Jinrui Saigo no Kirifuda desu!... tte, Maou ga Onnanoko no Baai wa Dou Sureba Ii no desu ka!?

About this manga

Five demons launched a war against humanity, destroying more than half of its lands. Among them, the Demon King, who has killed more humans than any other, and a duke's daughter enter into a political marriage, bringing the war to a temporary ceasefire. Cecilia, the duke's daughter, sets out to win over and ensnare the Demon King, hoping to prevent the extinction of humanity. However, the Demon King is hiding a major secret...?