First let me dispel the idea that a horror game is dark. While the dark can play an important role, too much of it can kill the game. Sometimes, things seen, or partially seen are more scary than things not seen.
Sound is very important in horror games. I would point you to the masters work (search for Umbra, by butters) for more on that. The echoed voice of a little girl can send shivers down one's spine, and the sudden loud noise can add greatly to a visual effect.
Timing is also critical. Horror is a dish best served quick and unexpectedly. Building, leading music is very useful as it builds to a fevered pitch, just before an expected event.... then delay to make the player wonder if it's coming before laying it on him full force.
An unknown or disorienting environment, eerie music, unexpected characters and events.. all add to the thrill of play.
Best.
I'm sorry, my answers are limited. You must ask the right question.