This is possibly one of the most important bits of advice for running horror. It is all about pacing the adventure/scenario. You start trying to build a general sense of unease, hinting that things are not quite right. Giving clues as to what may be wrong. It could be a bartender warning you not to step off the path at night or a community noticeboard with five different posters for missing dogs.
The second stage is dread, now the characters know that something bad is happening, and they dread it happening to them. At this point, it is still undefined, and indefinite. Is that black dog a grim portent of death, a rabid house pet, a werewolf sighting you as its next prey, or a vampire’s faithful companion? Or just a loose dog?
Terror is the moment when the dread that something could happen, becomes real and happening now. The source of the terror is closing in, cutting off options, it is coming for your, and it is coming now.
Horror is very short-lived, it is a moment. The big reveal where the werewolf bursts through the window, the vampire reaches out of the shadows, or your faithful hound turns to look at you with foam around its jaw.
The Horror is short-lived because once the truth is known, a player will know what they are facing and will go into planning mode.
The trick is to go up and down the levels. When you could have the werewolf attack, have the sunrise, and the wolves slink away. When the vampire could reach out and touch you, the local priest knocks on the door to see if you are alright, and the mood palpably lifts.
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.