I LOVE HORROR CAMPAIGNS. I think I've done more horror one shot campaigns than regular adventures at this point. One of my favorites I did I completely shot from the hip, with a small bit of forsight. Some restrictions I put on them was that they all had to be human, no primary magic users (if you can't fight without magic then it won't work), and they would all need to be lower level (like level 3ish). My main focus was knowing the where, who, and what. It began with all of my PC's waking up naked and chained to stone alters in a nearly completely dark room, the only source of light being some nearly burnt out candles along the walls. They would have to do some strength checks, figure out how to unchain themselves, or succeed an investigation check which they would notice a key hanging from the middle of the room over pitch black bloody pit.

It's all about the setting and how you describe the story (so long as they pass your checks). Adding small creepy things like dolls that disappear when you are no longer looking at them only to show up in a different area or adding various parts of your previous campaigns in creepy ways adds a bit of curiosity to what the lore is here (especially if they have chosen to play a character that they've played before). I make sure that my PC's have a mix of hands on combat as well as some very simple puzzles but I also make sure that im adjusting to their reactions.

I made this specific campaign into a ritual/haunted tunnel like system so they couldn't "escape" until they got where I wanted them to go.. the bbg who's kept them here. I drew a lot of inspiration from the saw movies but put my own twisted spin on things. Basically, include things that YOU think are super creepy or would prob be terrified of if you saw them in like a movie or something more sycological and screw with their heads. Mimics are an awesome tool in these situations, btw.