Category: Fear of the Unknown
Darkness is one of the primal fears of mankind. Night is a time when our vision is nearly useless, making us extremely vulnerable. This aversion to darkness is so prevalent that our cities and towns spew forth light at every opportunity, to the point that we have to actively work to cut down on light-pollution.
When dealing with darkness, two things must be kept in mind.
The first is the effect on the senses. Vision is our primary sense, and helps provide context for everything else we notice with our other senses. Take it away, and the most common of sounds suddenly become strange and unfamiliar. Something that normally feels cool to the touch will suddenly feel slimy and unbearable to hold. The movement of air across the back of your neck engenders terror and the feeling that someone just touched you. Could you really recognize a smell if you didn't have the visual context to guess at it? Even taste can change; we're used to seeing our food before it goes in our mouth. Not knowing what it is changes the entire dynamic of eating: where before we knew exactly what to expect taste-wise, now we're completely unsure.
The second effect of darkness is our complete lack of orientation. This ties into our vision, which we rely on for navigational clues. Without it, we're literally "in the dark", and any movement we try will be fraught with peril. There's a small object that you'd normally step around or over? Now you trip over it. That coffee table that you normally walk around easily enough? Now your shins bash into it. You stub your toes constantly on all those fiddly bits sticking out from the furniture. You misstep and fall because you weren't expecting the step-down as you walk through a door. You can't keep track of where things are, because the visual landmarks you'd normally use are completely unavailable. Darkness can turn a familiar, comfortable location into a dangerous obstacle course. All these effects are only intensified in someplace completely unfamiliar. Even with a bit of light to illuminate a room, one must be extremely careful in how they move, because the shifting shadows often hide the aforementioned dangers.
Those are just the practical effects, the problems that everyone has to deal with in a dark environment. But adventurers have to deal with worse. For them, there is always the danger of something hiding in the shadows, waiting to pounce at just the right moment. Something might grab you as you walk past that dark doorway. The monster might be creeping down from the ceiling behind you, or reaching out from underneath the bed. Better not make any noise, or you might wake up the fearsome creature sleeping in the attic. Is the house just shifting in the wind, or was that someone moving around upstairs? Was that a faint scream in the distance, or just the wind blowing through the trees? There's a reason we fear old, abandoned houses. The absolute darkness can hide many dangers, both mundane and non.
To scare your players with darkness, cut their information to nearly nothing. Force them to move slowly, or point out that they're making a lot of noise. Throw all those creepy middle-of-the-night sounds at them. Then, right when they're utterly and completely paranoid from all the false-alarms and unsettling descriptions, throw the monsters at them. The results will be much better than if you just threw a monster at them.