A Ravenclaw Genius!Harry that is kind of dissapointed with Hogwarts not teaching stuff like extraplanar travel, summoning, etc...(yeah, he's a D&D nerd) so he delves into some books he finds in the Chamber of Secrets and manages to contact a demon(more like a devil), she (the form she takes) can't really come through because the Founders and later Merlin placed anti-planar travel spells and enchantments on Hogwarts up the wazoo, but she tempts Harry and gives him lots(for him) of forbidden knowledge, she can't corrupt him because his moral fibre is unaturally strong(maybe Lily's love shields him somewhat), but she slowly makes him forget his ideals in little ways(why should those rules apply to you, power makes it right, you need power to defeat Voldemort), makes him trust her and even desire her when puberty kicks in. By Fourth Year she has convinced him to prepare a ritual so she can come over(away from Hogwarts, since Harry can't break the enchantments), enters Fleur DeLacour, a Veela who knows the old Magic and deduces what is happening to Harry, desiring to help him because of past experiences her family had with demons, and we get an Angel(Veela) and Devil on the shoulders situation.