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Nightmare Fuel: Knock-Knock
Even the basic Guests are especially creepy. Take one of the most popular ones - the first trailers showed it as a tall man in a straight-jacket and a very crude prosthesis instead of a leg. In the actual game he's basically the same but DOESN'T HAVE A HEAD.
The voices that talk to you anytime a breach is left open. It's debatable which ones is worse: the ghostly female one or the otherworld male one.
Or perhaps the crying child.
The diary pages that appear after the "Eye" segments describe odd party games that sound plausible in execution, and always end up meddling with the unknown(and the Invisible), to different degrees of Paranoia Fuel. One of them gets extra creepy, though, when it starts with you getting out of the house and coming back, and then finding the same table ready with plates and candles that was supposed to be put by yourself to invite Guests in. In other words, you are the Invisible.
One of them is a rather average one (for Knock Knock, that is) until at one point, if you here a voice speaking, it tells you to stop the game immediately, because now The Invisible is there.