Basically, at the very start you want blank space. Put dark colors in there, then shade the UI with a progressive alpha circle. Start your scene with just walking. The player will know something is up. Then, just when they don't expect it, BAM!!! Hit them with something scary. Scary noises, jumpscares, flashing lights, etc. Those are all good components. Basically, you can convert any game into a horror game. Just add these elements. Also, good horror games need a twist...