What about these titles?
-Shadowman
Insane atmosphear imho. Especially Playrooms...
http://www.youtube.com/watch?v=Dsj9mGHlqSA-Thief 1 and 2
Apart to be incredible games, the atmosphear is amazing and dark! Also the cradle level of the third is "powerful".
-Stalker (all 3, especially modded)
In a stormy night, being chased from a chimera is not good... You sense the danger... And you are in the Zone.
- Alien vs Predator:
the first one: insanely fast and dreadful game... Not easy that a game is so fast and scary.
Nightmare House 2
-a source based mod for Half Life 2 episode 2, very dark, scary and nice to play.
(i still have to look to Cry Of Fear mod for HL1, it should be dark and good too)
I want to mention these 2 games, not scary or disturbing but dark and with a wonderful atmosphear:
-Severance: Blade Of Darkness
Masterpiece combat system, truly a wonder....
-Arx Fatalis
There are some nice undeground dungeons. I've never finished it, but i plan in the future...