Once I'd got that in place, blamm, I sat down and the rest of the chapter just rolled out. It's fascinating to think about how that works.
I think it's because of genre. In modern horror, you're mostly dealing with scenes with just a small group of characters in a small setting, which simplifies things so you can focus on the horror special effects, whatever they are. It's about the personal fates - I mean, the gory deaths - of a handful of characters. I think.
A fantasy scene, though, has a big worldbuilding-y background that always needs description, and adventure fantasy means lots of characters and battle scenes and the fate of the world at stake. Jamming that all in as well makes it impossible to focus on the horror in the same way; there's just too much that has to be described and kept track of. Also the mood is completely different. That's probably even more important.