The Duke’s Daughter’s Mission to Seduce the Demon King, Humanity’s Final Gambit: A Political Marriage with the Demon King! …Hang On, But What am I Supposed to Do If the Demon King is a Girl!?

Synopsis

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...?