Everything about Amnesia.
Wow, this girl I know has been on about it's the first game to actually scare her.
Anyway, I actually have a story about this:
I was playing
Medal of Honor: Underground alone, the very last secret mission. I had gotten to the point where the game forces you to leave the prototype robot soldier guy you built behind since you must crouch in a lowered vent type area. I decided not to go, and my robot slowly patrolled the small area just before it. There was this large, blue empty room just by it that had a spiral of flat walls to nothing, the walls not extending to the ceiling to make it a spiral corridor. My robot kept walking down it, then back.
I had run back a little, looking down the hall to that room at a 90 degree angle. My robot slowly walked out of the room as I was saying in real life something like "Come on, my little robot friend." Suddenly, he turns around quick as forget and a rocket blows him to stuff. I'm flipping out like "OH GOD, WHERE DID THAT COME FROM?!" and rushed in... There was no one in there... Just a small spiral room... Creeped me the forget out.
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Edit: The Gameplay section of the Wikipedia Link I provided talks about the secret missions I was talking about. that whole secret mission was freaky as loving hell.