Isabella Brieris is a Demon and has been a loyal servant to the Phantomhives for years, but after a great tragedy, what will happen when another Demon comes into the picture, and not only holds a deal for the soul of her young master, but also made it it his mission to turn her into his mate? (This is a Black Butler series rewrite Fanfic of Sebastian x OC reader. I do not own Black Butler. All original characters belong to the creators.)