- Not being able to hide behind my weapons. Example: a really powerful and quickly regenerating creature that takes its time coming after you as you perform time sensitive tasks, like getting through a series of locked doors, gathering loot that you want, or figuring out a puzzle.
- Enemies making themselves present without being directly in front of you. Example: Howls of a group of enemies echoes through a dungeon every once in a while and gets louder as you get closer.
- Noises of things that aren't there when you look.
- Things noticeably looking at you out of your reach, such as red eyes in the darkness on the ceiling, but don't necessarily attack you.
And the most important thing with all of the above: Ambiance, ambiance, ambiance. Either the right horror music or no music at all + echoing and indeterminate sounds followed up with limited visibility.