When the thing you should be scared of is hiding in the dark, we need other ways to describe the scene.
We tend to rely a lot on vision when describing our scenes and to a lesser extent hearing. With horror, you can do a lot with bringing in other senses, both internal and external to the character. Smells of blood or decay, sudden temperature drops, or things feeling damp or cold to the touch. Internal feelings can be particularly creepy, feeling something moving under your own skin or wriggling inside you.
One of the most memorable scenes in cinema must be the chest-burster from Alien, and that starts with internal feelings of discomfort for Kane, which turns to pain and we all know how it ends.
Using internal senses is a great way to make your players cringe, particularly if you slow time down for the character to give you time to describe the worst bits!
These tips are taken from my Scarier Horror Play book. You can get 40% off this book at DriveThruRPG using this discount link. You can get 40% off all new releases as a loyalty discount if you join my contact list.