Okay, first off, don't add any weapons. In order to make the game scary, add ambience, dark areas, puzzles, jumpscares, but not too many. You should make the player find objects and other devices in order to enter other areas. Also, you should allow the player to hide in lockers, under beds, jump over opsticals, and run. However, every horror game needs a story line, and a villain that no other game has featured, not unless you want something like Slender Man or Michael Myers.
(Michael Myers is a good idea).
Plus, give your character a body, so if the player looks down, then you can see the character's arms, legs, and chest. That includes realistic, high-quality graphics. As long as you give the game all of the technics I said, then you'd have a unique game. Also, don't forget to add chase scenes, flickering lights, blood, or something like that, and the story line, well, you'll need to make that up by yourself. Trust me, because I write horror novels. I would also recommend a large map, and make the game long, not short, so the player gets a longer thrill. You can make the game supernatural, psychological, or paranormal. And, a good idea is to make the character more interactive with objects, such as computers, video logs, or maybe even lost tapes. Something to make interesting.
Although, it would help a lot if I knew what your story line was, so I could give you more details and tips. Anyway, if you have questions or comments, please reply to my comment. Remember, give thought into the game. Thank you! Bye.