it's a game that will not let you play it unless you boot it up within 5 minutes before midnight, and won't start until midnight comes, and you have to survive for as long as possible in a mansion (w/ nearby forest, and catacombs) and the game will mess with your computer while you play.
I think there's a trope for that. That'd be a good addition to a lot of psychological horror games, actually.
Why not make a game about being trapped inside a giant stompy bio-robot-suit that's slowly driving you crazy, and you're forced to kill all your friends because the giant stompy bio-suit is making you crazy and aggressive?
Or you could go for something more surreal.
I think that might be straying a bit too far into sci-fi for my tastes, though it would make a cool game. You could have it actively screw with your characters perceptions, so the player is never sure if those zombies are evil or just his teammates.
I... hesitate to watch that video. The name and YouTube's thumbnail make me think my mind might not be able to handle it.
As for the game idea, I'll throw in my suggestion. Don't define the monster. Create the game so that the player has to imagine the evil lurking in the shadows, but don't actually design one. Horror movies/games are always more terrifying before the evil is revealed. Have eyeballs floating through the shadows, tails whipping vases off tables, shadows of spiders on the ground that turn out to be nothing when the player looks up.
That would definitely be something I would try to put into the game.
Adding on to what the others have said, a game of insanity and paranoia. You hear footsteps behind you, shadows looming up, suspensful music etc. Basically think of those scenes where the player expects something insanely scary to happen but extended.
Same as above; that adds an extra element of psychological creepiness.
Adding onto that idea why don't you base the game around not turning around. Your goal is to reach the end of the level/game without looking behind you otherwise you die/lose. Your only incentive to turn around would be sounds, things you see in a mirrors, shadows etc.
This honestly makes me think of
SCP 087. That said, it only adds to how terrifying a game based off of this could be.
You're a child coming home from a friend, except it's dark and vaguely 90s so you don't have a cellphone. Go nuts modeling a frightened child's imagination in the darkness. Bonus points if you can work some parental horror in there too.
Should work fairly well with what others have already suggested.
I had an idea somewhat reminiscent (eh, kinda; it involved a little kid) of this a while back: you play as a child just at home. During the (short) daytime, you do little-kid things; playing, making messes, etc. Each night, you wake up and have to face a fear inspired by what happened earlier that day (for example, playing hide-and-seek with your dad would result in jump scares from closets and behind windows; if you had a birthday party and your parents hired a clown to entertain the kids (these aren't very good parents, are they?), you'd have Pennywise hiding under your bed and following you around). You can't find your parents at night; there bed is just empty if you go to check (in reality they're still there, but the kid is living out nightmares, basically). When morning comes, everything is back to normal, but the kid you play as might develop phobias and conditional panic attacks if you don't play well.
If you
do get caught by the monster, you scream and your parents wake up, run into the room, and turn on the lights before sending you back to bed.
However, the little-kid-nightmare game is bigger than I would want to start with for this sort of project. What I think I'm going to do is basically a The Last Apprentice game of sorts. You and a few other people online are running around a haunted area trying to take down malevolent entities. You have to work together to set up traps and survive, because the game is decidedly less in favor of the protagonists than, say, Ghostbusters.
Instead, you'd be facing off against enemies that vastly out-power you. Each type of enemy would have limitations based off of things like the suggestions you guys made: one might only be able to attack you if you turn around to face it, so you'd get paranoid if you think it's behind you (if it is, you'll die horribly if you turn around). Others might be able to pass through mirrors, leaving them if another player makes the mistake of looking at their reflection (sort of a reverse vampire; they can only affect you through their reflection). Depending on how many types their are, there could be a few more (one that is weak, but can possess teammates and act natural to slip past the other players' defenses, maybe).
The protagonists have a few tools in their arsenal, most of which are either based off of real-world superstitions or dependent on the type of monster being fought. Superstitious defenses could be salt, iron, or rowan wood, while more enemy-specific ones might be as complex as using a team member as bait to lure the enemy into a binding circle or as simple as smashing mirrors before the monster can step out of them. I haven't thought all the way through the protagonists abilities from a mechanical standpoint yet, though.
Multiplayer would probably be similar to Damned. One or two enemies, 4-6 medieval ghostbusters.
EDIT: Any other enemy suggestions would be appreciated, as well as possible means of neutralizing enemies.