I've done some brainstorm, and I don't think I have seen any of those two ideas on the site :
- A dark tunnel, located inside some other building. If somebody enters inside it, they will find that it stretches far beyond that what would be possible considering the outside architecture. After a while, all sounds and stimuli from the outside will cease, they'll be unable to go back, and some kind of humanoid creature would appear inside of the tunnel, resulting in [DATA EXPUNGED] for the victim. I know it's still a very "raw" idea, but I think that something effective and frightening can be made out of it. Alternatively, the creature could lure the victim inside the tunnel, compelling it to walk until it's too tired to go back. To lure the victim, it would mess with their perception of light, signifantly "darkening" the victim's environment.
- The "uncanny valley" : a geographically small zone containig a small human community. The persons born inside this zone possess slightly-altered physical features, making them seem "off" to outside observers. They also possess some limited memetic powers : every "normal" human observing them devellops increasingly strong psychological discomfort, ultimately culminating in murder attempts on the "uncanny people" or in suicide. Their dangerousness would come from their memetic effects, and from their rather extreme methods to stay safe and hidden from the rest of the world.
Be kind to me, these are just my first two ideas, and I know the probably suck ;_;