I've never really been a big fan of Halloween and horror. I kind of like it, but in small doses. I prefer my horror as a form of foe you cut down, or creepy evil that you ultimately deal with violently. Or at least magically.
All of that said, though, in a campaign way back when I did write an encounter that I never got to use centered on a horror theme.
After listening to the album "Welcome to My Nightmare" by the incomparable Alice Cooper, I wrote up an encounter based on Vincent Price's character in "Devil's Food."
I mapped out a little tower where an old wizard obsessed with spiders lived, gave him some appropriate allies (large and small), some spider-themed magical items, and a fair bit of loot to reward the players for discovering and eliminating this cover threat to the city.
I planned to drop it into Waterdeep since I was running a campaign set almost entirely in and around that city using 1st edition GURPS. Sadly, though, the PCs turned away from the adventure seed I had planted related to it and ended up going off for some adventures in the north.
In retrospect, I probably should have worked a Steven-like kid into the whole thing for some real upsetting foes, but hey, live and learn.
I know at least one of my players - the one who loaned me his Alice Cooper records way back when - would love to encounter this guy now. But the notes on that are basically lost in old Wang Word Processor files at best, gone forever at worst . . . and I've have to seriously, seriously up-gun the threat and wildly change the location to pull it off in DF. I mean, we've obviously still have spiders, but maybe not a Vincent Price inspired wizard along with them.
Or we might.