look at some videos of Doom 3 to be inspired.
Palying this game in complete dark with headphones was scarry as hell !
Atmosphear, scarry monsters and their sounds, voices etc ...
A really scary game that can inspire you.
One factor was that you didn't have time , and must go fast when ennemies was near for example, some of them they didn't put hours to reach your position : this was some stressing factor also !