A middle-aged man goes to the mountain to commit suicide. On the way he finds a sealed youkai black fox girl in a small cave.  She asks him to free her, and he does it without a second thought and goes on to die, but the fox girl, despite her original intentions, forces the man to give up and they eventually marry.
This is about their married life.

French / Français:  Un homme d’âge moyen se rend dans une montagne pour se suicider. Sur le chemin, il tombe nez à nez avec une démone renard scellée. Elle lui demande de l’aider, ce qu’il fait sans hésiter avant de partir mourir, mais la démone l’arrête et finit, suite à certains événements, par se marier avec lui. Ceci est l’histoire de leur vie de couple.
Links:
Author's Twitter
Author's Pixiv

English:
While searching for a brave candidate to become a magical girl and defend the world from evil, a mythical creature finds Kayo Majiba, a cute young lady who seems like the perfect fit. But when this newly turned magical girl proves to be a crude delinquent who goes berserk in a conflict, it becomes clear that this may have been a terrible mistake. Now evil—and any innocent bystanders—will face the rage of a magical girl gone wrong!

Portuguese / Português:
Enquanto procura por uma candidata corajosa para se tornar uma garota mágica e defender o mundo do mal, Myu, uma criatura mítica encontra Kayo Majiba, uma jovem bonita que parece a garota perfeita. Mas quando essa garota mágica recém transformada se mostra uma delinquente grosseira que enlouquece em um conflito, fica claro que isso pode ter sido um erro terrível. Agora, o mal, e qualquer espectador inocente, enfrentará a fúria de uma garota mágica que deu errado!
Links:
Author's Twitter
Official English release on Amazon