Pyo-Geol, the eldest son of a once-wealthy family now in decline, needs an heir to restore his family’s fortunes, but for some reason, his wife has been unable to conceive for several years. Pyo-Geol considers adopting a child for his wife’s sake, but his father insists that the child must be of Pyo-Geol’s own blood. Unable to bear it any longer, his father proposes a secret strategy passed down through the family for generations, which involves conceiving a child by engaging in a relationship with a monster hidden within the family.
Will Pyo-Geol ultimately follow his father’s will?(Source: Ridibooks, translated)
— Description from AniList community (anilist.co).