Story Idea!
I did base this idea on your past story "home before sunset" and another story I remember reading a while ago.
So basically a human x wolf demon romance story where a young human lives in a kingdom where the people are taught to fear demons, and told they seek to destroy and corrupt everything in the surface world.
But the demon king, a large and imposing pitch black wolf, has been unknowingly trapped beneath the kingdom in an old forgotten temple, restrained by a magic seal for centuries.
One day, the human finds himself lost in a cave system beneath the kingdom and finds his way to the old temple where he meets the demon king, who, having finally been given a chance of freedom, asks the human to scratch the seal to break the spell and free the demon, promising the human that nothing bad will come from his freedom. After some more convincing and a bit of flirting, the human hesitantly breaks the seal and sets the demon king free.
Being a demon, he lied, and after taking a liking to the human, decides to mark him as his mate, and kidnaps him and brings the human to the underworld alongside him, and he is no willing to let the human go anytime soon.