I like the movie The Mothman Prophecies, but I rarely watch it.
The same hold true with terror in RPGs. A momentary fright is one thing, fear and suspense is another, and even horror can be more than appropriate.
Terror can push things too far. Don’t expect your players to come back to the table afterward.
Terror happening to the characters is perfectly fine.
But ff you really want your players to feel terror in your game, then you better damn well have some kind of catharsis at the end of the session.
No cliffhangers. Wrap up the session like a movie, not like a season finale of The Walking Dead.
Terror changes behavior. Sometimes permanently. It triggers flight or fight.
Terror gets under the skin. Makes you not sleep well at night. Gets you thinking about things in the dark.
Ever read Stephen King’s The Stand? Try sitting next somebody who sneezes afterward.
Ever been to a vampire LARP where they take things too far? That’s terrifying (and stupid), (and it’s why I don’t LARP).
Ever been to a RPG where the GM likes to terrorize his players? You’ve probably gone once, and never again.
Some scenes in The Walking Dead are a notch worse than the fear I experience watching The Mothman Prophecies. But I don’t find The Walking Dead terrifying, frightening at times, gross, of course.
Some readers will probably disagree.
And players may be terrified by different stimuli than others. I know a gamer who can’t stand spiders. Terrified of them–even in painted miniature format. So I’d think twice before putting spiders in my game.
Even horror can push certain limits. Terror breaks them apart.
Use techniques of terror in your game with extreme caution–and only rarely.