Nightmare appears after you escape her (and with the book possibly) but apples are your best friend here for bella just don't get caught so much..
Viewing post in Escape From Deep Pit University comments
I was hidden and crouching behind a solid wall in a bathroom after evading her when she saw me from the other end of the hallway and still ate me. I saw her belly since she ate a student and nothing else when she spotted me and despite the fact I was crouching and hidden she still ate me even though I was well outside her view when I hid. For some reason she sees me through walls after I get spotted and will always catch me even if I should have obviously escaped. Not to mention more than half my attempts had all apples behind locked doors that had a key for them behind a locked door. I'm fine with RNG games so long as there is leeway for mistakes but the RNG in this puts the player at a massive disadvantage and unlucky people like me are basically never progressing or unlocking anything. I like the game a lot, but the preds can see me from way too far away and only give a hint if they are very close.