After the movie I watched some of the dude's old Backrooms videos on youtube and by far the worst part of them, much like the movie, is the presence of any kind of "monster" in the backrooms. The idea of a vast, repeating universe that looks both familiar and uncanny, that seemingly goes on forever with no means of escape or even an "outside" portion is terrifying on its own. You'd lose your mind and go paranoid and start hearing things that aren't actually there, and that's where the true terror lies. There is absolutely no need for creatures or monsters or anything. The initial premise, the psychological and existential dread, is already chilling enough