To be honest the best horrors whether it be games or films are the ones that make the player/viewer scare themselves, you just give them the push lol. You really need to do things that get into people's heads. Here's a few examples/tips.
MusicQuiet eerie music is great for building suspense, even just the soundtrack of a storm outside or wind whistling is good. Pianos can be quite creepy if used correctly, would be good if you could make it out of tune slightly.
Dissonance is a good musical trick to take advantage of, this is the use of unnatural sounding tones together, think of Hitchcock's famous psycho track when the lady gets stabbed, two tones a semi-tone apart is not music to one's ears and can cause the listener to feel edgy, like nails on a chalkboard. Using this well could create a solid creepy feel.
Ambience -
Don't be afraid of silence - Although music can create atmosphere, the lack of music can cause the player to become anxious if the atmosphere is already set.
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Make use of lighting - Making things look scary is cool but people are more commonly afraid of what they can't see rather than what they can. Try using lighting effects to create dim patches or completely pitch black areas. Flickering lights help lol, you can show them something scary for a second and then they can't see anything. What's going on!?
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Make use of sound effects - If you're in a supposedly empty place there's nothing freakier than the sound of something that you can't see. Whispers or creaking noises. Perhaps a door opening and shutting at the flicker of a light, the sound of someone else's footsteps, all these things can cause paranoia or chills up one's spine
I was going to mention the visual side of things but I think I summed what I would say up with lighting. To sum up though I would say that keeping them guessing is always good, you always know in a movie when someone is going to jump out because of the music, try to do that sparingly otherwise your scare moments will be too predictable. Actually that's good, unpredictability is a good thing to keep in mind

I feel like I just wrote a tutorial on this topic lol. Never mind. Hope you find all that useful

I'm off to bed lol.