I know my way around Commanblocks very well and would like to "build" a horror game mode myself, I would have any ideas, I'm open to everything.
Jumpscares where, for example, everything goes black and a grinning face comes to you and a loud cry of pain. But I think I had to look for something copyrightless from the Internet so that it also scares you. And, make places where everyone expects a jumpscare but no one comes and then make places where one comes out of nowhere with the creepy atmosphere. And make zombies with really creepy faces that are very fast and a typical zombie animation so that it looks more like a zombie and not like Minecraft. Different zombie faces would be good, so it's a bit like Outlast where you also encounter different creatures. And just random step sounds when you are on the stairs or in the building. Or somewhere inside.
Edit: I just noticed that this is more of a map. But you can add a few features like the one with the zombies or buildings or something. And you can make normal mobs creepier.
Use / particle with execute for certain mobs for blood particles (but suitable is e.g. The block particle redstone block…)
