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

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...? (Source: Young Champion, translated)