Not a bad start, though I think it got just a little too stagnant as it went on. I quite liked the ending portion, though, and personally I would lessen the time it takes to get there. You're already using marcato (or is it sforzando?), which is great, though you could build on the feeling it creates even more by raising the volume, adding more instruments, and increasing the buildup to it.
This strikes me as the sort of thing that could make use of multiple layers of sounds. Counterpoint would also work quite well, I think. Hell, I'd even say to go for polytonality. Or tonality within the context of chromaticism. Now
that would be awesome.
It sounds like you have a pretty decent (or at least functional enough) string library so it likely has other sampled techniques like pizzicato that you could make use of. I would also distinguish each part more. You've got strings, so why not use them each to their full potential? True, it's more difficult but your music will be richer because of it.
You may also want to add more unconventional samples. Having strings is perfectly suitable, don't get me wrong, but think of how much more uneasy it would make someone if they had absolutely no idea what was making the eerie sound they're hearing. You could accomplish this was ambiance patches, custom timbre, etc.
You can do a lot to further play up the nightmare aspect. Consider adding more minor (or major) second, tritone, and other dissonant chords and generally introducing things to surprise and unsettle the listener. For example, after a brief pause someplace you could have the violins start at fortissimo, sounding like they're attacking the listener (probably also adding some marcato articulations). You could include a calmer middle section, as well. Doing this, especially after really ratcheting up the tension, can really help keep the listener from predicting what comes next and lulling them into a false sense of resolution. Then, of course, you would then drop them right back into the tension but this time ramped up. That's what I would do, at least.
One last thought I had was to change the tempo often, possibly even going from one extreme to another. This may not end up sounding that great but I'd try it out.
If you need some inspiration for what to do next I would direct you to Penderecki's
Threnody for the Victims of Hiroshima. It's one of the most unsettling pieces I've yet heard.
But like I said, it's definitely a good start. You should update this thread once you've finished it. I'm interested to see what you will do with this.