There are a lot of tools in a horror GM's toolbox, but description is one of the hardest to master. I've been GMing horror games for... better than 25 years, now that I think about it, and description is still something I hone and refine every session.
So I don't think anyone is in a good position to talk about it from on high. It's something we're all working on, laboring towards an unimaginable goal. But it's a good reason to have a conversation.
One of the first pieces of advice I always get is to evoke all five senses. Smell often comes up here because it's rarely used, but the real point is to move beyond just using sight and sound. Smell can evoke many emotions, but touch and taste are more important for many kinds of horror. A hauntling may smell of grave dirt, but the chill feels all the more personal.
Even changing that verb can make a huge change. Telling a player "you smell blood in the air" will evoke a different response from "you taste blood in the air." Part of that is because, well, it's in your mouth. But just as important is that most of us have tasted blood while not having been around enough to smell it. We know what it tastes like, but often only have descriptions of what it smells like.
Evoking those memories is part of the strength of good description. You're not even tying the horror of traumas we've survived in the real world (a dangerous trick), but tying horror to the player's relatively innocuous memories.
And that's one reason that horror RPG description could be its own weekly series until the end of time. Far more than even a Stephen King short story or a horrific board game, the GM in a horror game can craft personal horrors for the players.
You can learn what horrifies your players and what just grosses them out, along with what buttons to push and which not to. It gives a tremendous flexibility and power that doesn't exist in other venues, both to scare players and to create a safe environment for people who can't normally experience that in the wider world.
It can get lost in the descriptions of roiling masses of unclean flesh and the feeling of things crawling under the skin, but horror GMs have the flexibility to create a really beautiful and unique experience for the players you can't get anywhere else. And that, I think, is part of what makes it so worthwhile to hone our art.
Next time the sun goes out,
Landon Winkler, Shadows over Vathak Line Developer