this character in a previous iteration was only a novice at witchcraft, mucking about with things she didn't entirely understand in an attempt to resurrect her sister. maybe she opens some kind of hell gate by accident and unleashes demons that need to be forced back inside before it can be closed again?
i'd want to leave you with plenty of creative freedom for your character! you could play someone more experienced in magic who she goes to after the fact seeking guidance to undo her catastrophic failure. or some unwitting guy who finds himself demon-possessed. or anything supernatural in between. let me know if you have ideas or preferences!