Amnesia: The Dark Descent is a first person survival horror available on PC, Mac, and Linux. It is a game about immersion, discovery, and living through a nightmare made by Frictional Games.
If you are in the darkness too long, witness unsettling events, or stare at monsters, this will reduce Daniel's sanity (which needs to be maintained). If not kept in check, you are in for hallucinations that draw the attention of the monsters. Not fun at all!
Amnesia: The Dark Descent is one of the scariest survival horror games I have played on PC. Its thick atmosphere is terrifying. I love how well the gameplay is crafted. The scares are horrific and memorable. The story unfolds slowly to its unsettling climax. In my opinion it is a must-play, you do not know what you're missing.