You are studying a patient you are experimenting on, talking with him and observing him ("discovering his body") as he slowly morphs, seeing all of the changes he is going through and how he is coping with it. The premise of it is that you are some sort of doctor or scientist in a future where, I don't exactly recall the backstory, but some event has happened, and you are testing on people to turn them into fungus/human hybrids (voluntarily for the greater good, for some reason), and through this process, humanity can form some kind of hivemind through that mycelial network, or however the game explains it. It is very funny, but very gross, and it really stuck with me in a way most horror games have not. I never played it, but I saw the game on youtube. There is one little indie horror game that I wish I never ever found. I thought of a good answer while typing this out. Both are pretty effective at startling me and getting my heart going, but they are unremarkable otherwise, you know? Horror movies are fine, horror games are quite difficult because I have to interact with them, and not just observe them. I have played a lot of Alien Isolation a yr or two ago, but I think at a certain point with those types of games, you are less scared about the thing chasing you, cause it just devolves into silliness, frustration, or you are aware of the "gaminess" of it.
I couldn't get an hour into Dead Space 1. Luigi's Mansion scared the piss out of me when I was little. I might be trying to think too hard about this