In your typical jrpg-esque world, the newest hero destined to do battle with the demon lord is a young girl whose village was destroyed for having been unfortunate enough to be her home. Fueled by her desire for vengeance, she grows in strength at a fearsome speed and become infamous for the absolute lack of mercy she spares for demonkind. Her gradual conquest of the demon world proceeds smoothly for the human world until her knight mentor reveals to her that it wasn't the demons who destroyed her home but the kingdom she fought so hard for.
Obviously shaken by this, she readies herself to turn her blade on the ones responsible for engineering her tragedy but as it turns out her mentor is the last of those complicit in the deeds as the rest have perished over the years. Conflicted between her rage, her admiration and her duty she eventually decides to cut all ties with the kingdom and begins travelling on her own. Still, she pushes herself to fulfill her heroic duties of slaying demons but she finds her will to do so waning unceasingly until she makes a homecoming to the ruins of her birthplace where she realizes that while she understands that mankind as a whole did not deserve blame for her pain she can't help but want to put it on them anyway. With the complete loss of her reason to fight hanging heavy on her head, she falls into a slumber of an unusual nature.
She finds herself young and childish again in the village that once was so full of life. It was only a dream she realized but one so sweet and nostalgic that she allows herself to fall into its reality. It is only when she falls asleep in the dream that she finds herself waking up in the present in a strange room filled with magical instruments. A demon greets her there and before she can destroy him in frantic instinct, he explains that he was responsible for the dream she just had.
He tells her he's a magic scholar that specializes in blurring the lines between reality and artifice and that he can bring her dream into their reality for her piece by piece. Naturally however there is a catch. If she wants him to make that dream come true, she must subjugate the human world and seize the throne of demon lord to serve his own purposes. Furthermore, she must also trade away her humanity little by little for the magic to work. She is hesitant to accept though not out of an internal conflict but out of skepticism of this miracle he offers. That changes very quickly as he summons a familiar face out of the dark to convince her.
Face to face with a duplicate of her dearest friend, he tells her that alchemy and necromancy fall under his areas of expertise and that given enough time he will be able to call back body's actual soul and give her a second chance at life. In fact, he can do so for this particular soul immediately but she needs to make her first sacrifice of her humanity to make it happen. No longer caring anything for it, she agrees to his bargain and they begin the work for reunion