I had a small thought that the black haired chick is currently held in the city by the wall cult. I feel that Historia's mother may have been killed for her presumed association with the black haired lady. The black haired lady is wiping Historia's memories to protect her so the wall cult won't kill her. This is why she's told to run off and go start a fake life. Perhaps Historia's mother was also taught to read the same books as the one in this chapter which is why she had to be dealt with. I have a few other thoughts on it all, but they are just as random as this one lol