In a world where humans and demons are in an endless war... One day, the demon king, Gil, arrives in the Kingdom of Hazelbard, a large human territory. Gil proposes a "truce" to the humans on the condition that neither side will invade the other's territory. However, due to a scheme by the queen, who is also his stepmother, to credit the agreement to her son Clemon, Princess Estelle is forced to marry the demon king in exchange...

(Source: Cmoa, translated)