Amnesia is the scariest game i have ever played. by far.
let us take a scenario:
(may contain spoilers)
I was in this prison/dungeon, very very dark. Lack of tinderboxes, low on oil. So iam sneaking around trying not to be seen. I walk up the stairs and there he is, growling at me, I start running down the stares. I run and hide behind a rock, hoping he didnt see me run there. He stands in the middle of the corridor. looking for me. In the end he gives up and goes the other way.
I decide now is the time to start looking for a way out of here, since the way I came from is blocked. I search and i find a hammer, in a small cell. I walk out to the hallway and there it is again! the monster! There is no way to run! so i simply run back into the room and put the bed in front of the door.
Smash.
Smash.
and the door breaks down and in comes the monster, I grab a chair in panic and throws it at him! He staggers a bit, I try to run past him through the door.
Then I feel a huge pain in my back, he hit me! I make a run for it and manages to get away.
I find a small room, close the door and uses my last tinderbox to light the small candle.
I was safe once more. But for how long? I still needed to find a way out, still had to finish my mission.
I know, not the greatest explanation and not the greatest enviromental telling aso, but thats a part of the game, and i got scared as hell.
The most terrifying part was probably that you CANNOT KILL IT.

[Q]uik, Quiker than most