Vowed Upon Darkness

Fallen angels don't always return. Their love for mortals are forbidden and only dealing with a demon is the solution. When Percy is punished and sent to earth, he breaks the first rule and falls head-over-heels for the first mortal he meets. Being attached isn't a good outcome, he learns this as he shakes the hand of the vilest creature on earth. Nicercy! Based on a vocaloid song.