True End Of Those Days Manga

いつかのトゥルーエンド ; Itsuka no True End

"Marry me!" A world where the war between humans and demons has continued for hundreds or years... The hero was summoned from Earth to be humanity's Ace in the hole. Aoi Yamato was a regular university student, but led them to a sweeping victory and ended the war. However, the softhearted Yamato did not eradicate the demons, declaring that "the hero and the demon king will become a married couple" as proof of peace. Fast forward to a year after their marriage... Yamato and Demon Lord Rei are enjoying a lovey-dovey, newlywed life. Yamato is madly in love with his gorgeous and devoted Rei... while the serious Rei tries his damndest to be a good wife in all capacities.