D&D General Horror is hard

[Sorinth]

2) Don't try to outgore Hollywood.
The Saw films are terrifying both because of the underlying idea of being punished for what we largely perceive as minor failings of morality or social convention and as a jumpscare of gore. If you sit there and try to describe something hardcore enough to outgore Hollywood you'll have wasted 20 minutes on describing viscera without inducing horror, just nausea.

One thing to keep in mind is that if the PCs face off against the monster, it's almost always a fight to the death, and in theory the PCs should win. Which means you often can't introduce the monster directly until the climax, so the way you build up the horror of the monster is more often then not by the PCs encountering the aftermath of the monsters actions.

So while a 20min narration is over the top and can even be counterproductive, describing the horror, whether it's a grisly murder scene with lots of gore, or body horror, or whatever else is a very important aspect of doing horror in a D&D game. It's obviously difficult to find the sweet spot, especially since different players will have different sweet spots, but you are honestly better off going a bit too long then going a bit too short when describing the horror aspects.

BUT WHAT DOES THAT ALL MEAN!?
It means if you want to run a horror game you do it by proximity instead of direction. Rather than the Monster being the scary thing, the Monster is just "The thing that happens". The Monster is the Chekov's Gun. The horror is in where and when and who and how it shoots. No one watching Son of Frankenstein was gasping at the Monster and being terrified at the idea of a world where such wickedness could exist. They got that in the first one. Now the horror -surrounds- the monster.

In Curse of Strahd, Zarovich can appear at multiple points in the story while he still has all the power in the world above and beyond the player characters. He could, if he chose, kill them all as a CR 13 monster while they're level 4. But he doesn't. Mainly because the game would be really short and boring if he did... but also because that's not scary.

What's scary is Strahd flexing his power not by striking the party... but by landing VERY CLOSE TO HOME. By killing NPCs the party knows and likes. Oh, sure, it's an old trope to get players to hate a character... but it works. And you know what else works? Charm.

This somewhat covers the point I'm trying to make above, while the monster can and should be made somewhat scary in the final climatic battle, the real horror is coming from the thing's it's doing before that fight. Some of that can be the simple fact that an NPC the players like is/could be a target, but a lot is also the descriptions of what remains of that NPC after they've been targeted.