Kids these days!
On a roleplaying forum a discussion starts about horror RPGs and how you create fear. And everybody starts talking about game rules and mechanics.
Which is oh, so wonderfully naive, and misses the mark by a half-mile.
In a roleplaying game, we get players playing the roles of characters.
Scaring the characters is easy.
The Game Master says “Your characters are scared.”
Done.
There can be specific rules to simulate fear – the old Ravenloft setting used a Saving Throw vs Death and Paralysis or a Will check. Fail that, your character is scared. Other games used different formulas. Done.

But if one of the the purposes of horror fiction (and horror roleplaying is interactive, shared fiction) is for the end user to experience the frisson of fear, then the fact the characters in the stories are scared witless is not enough. We need to get to the end user – the player.
And here’s something I learned in my long life as a Game Master – nobody’s scared of a roll of dice or a table.