The Demon King's Beloved Wife Isn't Loved at All

In a world where humans and demons have been in constant conflict throughout history... One day, the demon king, Gil, visits Hazelbard Kingdom in the human territory. Gil proposes a truce on the condition that both races do not cross between each other's territories. However, in order to make this agreement her son Clemont's achievement, Queen Catalina forces her step daughter, Estelle, to marry the demon king.

(Source: MangaPlaza)