Huh, you call it a story, but then mention chapters (which I relate to novels), so now I'm curious how large you're expecting this to be (not that it really matters). Also, you say story game at the end, so now I'm extra confused, as there is no link to it.
As such, if it is a storygame you're writing, I would have put it on sneak preview and put a link here. Although, it seems it is currently not on sneak preview (or I'm just stupid).
Other than that, I should mention that horror is not my strong suit, so I'll just list some of the general advice here:
- Tension is good!
- A sense of powerlessness is a common tool.
- Getting readers to relate and think about what they are reading will help (shocking, I know).
Horror
Scaring people with words is an interesting thing, and two general places it can come from are concept and empathy. Or at least that is what I'm thinking right now. Anyway, rather than rambling, I'll instead ask how many horror books have you read?
I know horror books exist, so seeing how other authors make stuff scary would be a good idea, at least if you truly wish to 'give people nightmares'. Also, if this is something that'll be available online, you can use jump scares! Just set a loud sound to play and hope the reader doesn't have their volume set to zero.
Jokes aside, I think high writing quality (or very purposeful writing) is necessary for horror, as otherwise the suspense/tension can just deflate, as the reader is taken out of the experience. Keeping readers immersed/engaged would thus be paramount.
Learning a bit of the technicalities of writing might help with this, such as Gower's recent article: Cumulative Sentences, Part 1.
The rhythm and structure of cumulative sentences makes you describe fully, thoughtfully, pushing you, the writer, to return to your original thought, demanding that you think about both how the sentence sounds rhythmically and how much precision of description you are offering. It sounds amazing. It has, and I am not attempting to exaggerate for effect, a hypnotic pacing if done well.
Sounds like it could be used to great effect in horror, no?
Conclusion
Maybe bump this thread on the 1st, as the contest will be over, so more people would have time to spare. However, you do say you'd like people to take a look at the first chapter, so does that mean the 84k is just the whole thing so far, or is that the first chapter length?
Regardless, for horror, I personally think hashing out the concept thoroughly, and then getting a complete draft written, which can be revised and edited later, is the way to go. This might just be me, but I find horror extra tricky to write, so I think a drafting approach makes more sense, but hey, you'll know what works for you better than I.