This world is an extremely vast one, with very old rituals. A way of living so old, nobody knew where it quite originated, though everyone followed it fiercely. In this world the threads of fate are sacred, everyone has a role to play later in life each equally important. It was said very early on, someone devised a way to find out which threads of fate were woven in what child, thus able to determine what their future would look like.
However, that by using this method of 'cheating', people started taking matters in their own hands which in their eyes seemed justice. Pre-emptively murdering those who were deemed a 'threat' in the future. This quickly build up to cause a massive ripple and lead to a catastrophe across the vast world. To prevent this, enormous schools were made. Children were sent there to find out about their future and receive proper guidance towards this goal.
Funnily enough, this meant it was a place where current friends could come become future enemies, though that was for fate to decided. Thanks to the diversity, accidents still happened, however were reduced drastically thanks to a very strict system. Until they had reached their peak, it was mostly just like a regular school. Only with all manner of monster and magician living together.
One young man, a very optimistic one and extremely kind one, came to the school with big hopes. Though after the process, it was revealed he would be a Prime Incubus, a very rare being of extreme power. Whether he liked it or not, such a thing caused commotion among the students. Most of them afraid and others very much attracted to his future power.
Though the boy himself tried to deny it, and clearly was very much opposed to the idea of him turning into it. A strange occurrence, since most people become exactly what they always wanted to be.
The story itself starts right after a tragedy within the school, where over the course of a few weeks, a female has been creating a bond between herself and the Prime Incubus and eventually killed him. The story behind this, is that she was seduced and deceived by a lesser demon nearing his end of the school term and enchanted her to get rid of this Prime Incubus, a being who would be far above him in status.
Accidents involving murder were almost unheard of, though the school dealt with the lesser demon and the woman was excused due it being out of her control due to the spell. Though what many don't know, is the boy just so happened to confess his feelings to her before the knife pierced his heart. A surprise attack he had not seen coming. A week later it came apparent, ironically, her betrayal and deceit by use of seduction, actually enabled the change to form within the boy.
Due a Prime Incubus being so rare, many did know this demon was created thanks to a man being murdered by his love. Creating a great sense of betrayal and a high desire for vengeance. So the boy returns to class, as if nothing happened, only this time very much in tune with his nature. His confidence through the roof. For her, a constant reminder of what she had done, now worsened by the fact that she knows it was her doing that created him.
Because even though she had been deceived herself, she had been fully aware of what she was doing the moment she pierced the boy's heart. (Note; The female character in this can have whatever role you desire or you feel is fit; I am quite flexible, though of course that of a priest or nun would be rather bizarre and unacceptable.