Darkness is not what makes something scary. Pitch darkness in itself will not instill fear, it will only instill the desire to not move so that you don't bump into anything.
What always instills fear is poor visibility, but enough visibility to make your way around. That in itself isn't enough, though. Allow me to explain.
What makes a scary game scary is anticipation.
Now, this will only work if there is something TO anticipate. If your game has never actually done something startling to the player, they will not anticipate it happening. Show them a corpse ridden against the wall with claw marks nearby to give them the idea that a monster killed that person, or portray a basic creepy storyline to give them an idea of what might jump out at them. Without them KNOWING what to expect, they will not expect anything and the sense of fear is lost.
Also, you cannot simply spawn a zombie next to them all the time and rely souly on startling the player to make them scared. That isn't fear, that is simply a startling thing to do and becomes expected after a while.
You need to promote the sensation that something is about to happen, but design the area in a way that they are expecting it to come from ANYWHERE. If there's only one corner and they know something's going to pop out soon, they know it's coming from around the corner. Same with objects that things can hide behind, or holes in the ceiling. Don't give them a single place for your monster to jump out from, give them a thousand (exaggerated, but a lot anyway) so this way they are not prepared for the direction that something is coming from.
Now after that, don't make it so prompt. I scare myself the most when I'm expecting something to happen and it's not happening when I think it is.
Inform the player in a very subtle way that something is GOING to scare them, but let them sit for a bit and allow their imagination to build up fear. Tell them that something will jump out at them (not directly but make them ANTICIPATE IT) then pose several methods of it happening, yet don't make it happen for a little bit so that they're at the peak of their anticipation.
The art of scaring is not so exact, but it is always best to allow the person to make their own fear. Turn their imagination against them and bring it to a boil just before the actual startling event takes place.
On top of this, atmosphere
atmosphere ATMOSPHERE!!! Creepy environment, creepy sounds... the occassional object toppling over behind the player (and very occassionally have something scary actually be the cause of that object falling over, otherwise the player would not be startled by objects falling over)
Now go and make a scary game.
The one and only,
