This manga tells a story about a demon reclaiming what's belong to the realm of the fiends from a grip of a young mortal.
They are allowed to use certain methods, by any means necessary to kneel down their enemies.
TBH I really want the demon to just eat the young foolish mortal and life happily forever.
Just kidding, that's not the true story of this series. It is though, still a part of the story.