I feel that the core of any good horror experience is based on the emotions it leads you to feel, which is not always a sense of abject horror in all cases. Silent Hill 2 is a good example of this. Of course, frightening sound design and ambience and a solid visual atmosphere is incredibly important for any horror game, but I feel that there's a trait in some of the best horror experiences have which is seldom talked about, but the meta-ness of a given horror game. What I mean is, when the game makes you feel or wonder about the game itself and its circumstances, or its relation to the real world at hand.
The earliest sort of games I can remember sort of utilizing this were .EXE games/creepypasta-adjacent games which used to be popular in certain circles way back when, though everyone knew they were just games, the idea of software coming to life and haunting your computer or something like that was compelling. I even remember a Unity engine based Ben Drowned game that actually had some malware adjacent functions built in, and apparently would try to take pictures from your webcam, even some had the pictures emailed to them with their faces blurred out. But I never saw any receipts for that.
In the modern gaming sphere, the two games that I can think of in recent memory that really utilized this was notably P.T and Doki Doki Literature Club, with P.T being the better implementation of this sort of meta-horror since Doki Doki had a horror disclaimer at the start. I'm not sure if some of you will remember but, for a few days before anyone knew that P.T was a KojiPro work there was a bit of a storm. Here you have this amazing looking for PS4 standards horror demo thing that just gets disturbing and more disturbing. To this day I think those 3 days before it was widely known to be a teaser for a (potential) SEI Silent Hill game was some of the best implementations of meta horror. No one knew what this thing was, why it would break the fourth wall and show messages to the player from mysterious people, etc.
The sad part is this sort of meta-horror I am describing is hard to implement since a lot of it works on the player not being aware of certain elements, such as the game being a horror game, the creator of the game etc, which really can only be achieved if you choose to deceive all your players and start breaking the fourth wall "imscared" style with a marketing campaign or something or have a lot of clout as a developer or with an established IP like KojiPro/P.T being a Silent Hill teaser. In any case the next best horror experience that the internet will go nuts about will likely utilize this thought process in some capacity. And as an aside, eye tracking introduced into HMDs like PSVR2 gives me tons of ideas for horror game mechanics.
