I can't remember the name but I from what I remember that the FL woke up to have already eaten a few human beings maybe children as well but the ML was still alive barely. He promises himself he was going to get his revenge since he remembers all the times the FL had killed him but the FL in her old life used to work with kids (not 100% on that one) so she is nice to him and eventually uses her magic to become a kid to go to a school with the actual leads of the original story.
The FL was either a witch or demon.. she also had a group of like demons/witches who followed her not so big like 3-4 people until she disbanded it so they could be better people while she went to school.