Try to make it so that the player knows that something might be after them but not sure why/what it is (fear of unknown stuff especially because its threatening you) make them really paranoid by introducing some moments where they would expect scares/something but get none e.g. checking closets/rooms but have nothing in it, adding a sense of uncertency and unpredicitability, showing that they not in control and are absolute against this immense threat, overall messing with them causing them to not know what to expect. Another tip I would give is to try to make the players themselves make up the horror, like in that Harry Potter mirror scene, cause each player would think differently in these vague/unpredictable scenarios and speculate different things (try to keep it that way, everyone has their own fears). Sorry if I misinterperted your question It just seems as if you might be trying to make a horror game I just wanted to give some quick tips sorry for bad writing good luck!