Ayakashi Meoto Roman

Yuzaburo married Tsumugi, a fox demon 100 years his senior, in a political marriage that transcended the boundaries of race, between humans and demons. There was no love between the two of them, who registered their marriage as proof of a peace treaty, but... This is the story of a boy and a nine-tailed fox who are entrusted with the mission of "bearing a child" for the sake of peace.

(Source: Natalie.mu, translated)