Many of these will have their own previous required mods so always always check the Requirements tab. Some obvious good options, like Monster Valley, which turns the townsfolk into monsters, haven't been updated. (See here:
https://forums.stardewvalley.net/threads/unofficial-mod-updates.2096/page-106#post-81414 )
Spoopy Valley: Dark crops and trees
Spoopy crops, fruit trees, recipes, and extras!
www.nexusmods.com
Wallpaper, flooring and furniture mods (Spoopy rather than horror (Have I mentioned I hate the name spoopy? I do. I don't mind the aesthetic, but I haven't used any of these myself)
Felt like some good ol' weirdo wallpapers and floors were missing so I made some for Alternative Textures.
www.nexusmods.com
Wanted to add some more spoopy variations to the furniture so I did some!
www.nexusmods.com
Closer to horror decor
Witchy-Gothic Interiors:
This mod is not currently available
www.nexusmods.com
there's yet more decorations than these, But I don't know what is and isn't up to date, and I'm not linking to things made in 2016 and seemingly untouched.
Witchy Valley - Lets you make potions learned from the Wizard, adds some decor. Based on comments, seems to work at least recently.
Adds a cauldron, new potion recipes, and an optional witchy furniture pack.
www.nexusmods.com
Idaida's Gothic Buildings. Fairly new mod, should be fully up to date. Farm only, but very good aesthetic
Gothic/witchy look for all the farm buildings.
www.nexusmods.com
Enchanted Valley: A WIP that turns townsfolk into monsters or fantasy beings. Still a WIP, but the authors hangs around in these forums sometimes.
Add some fantasy to your valley
www.nexusmods.com
Ghost Town: Everyone is dead. (Seems to have been working for people up to this week, but needs PYTK.)
The villagers are dead, but they don't know it.
www.nexusmods.com