Looking for a specific idea

Background.

Our characters attend a school (college/university level) for higher beings. Anyone but mundane humans can join. Think of Hogwarts but more mature. Students end up in a variety of careers but at least half of them becomes Shepherds watching over humanity and protecting them from supernatural threats.

The story would actually start where he's a demon and she could be a witch or something along those lines. They fall in love after he seduces her but to their surprise she's become a demon herself and they've also become soul bonded. Afterwards they learn that a race of dark beings wants to destroy them so they can conquer the mortal realm and they have to deal with that along with their responsibilities as students.

There's more to it than this and it would be a long term story.