This is a pretty sweet track.
However, I have a couple things to critique. The synth, bass guitar if I am hearing right, that comes in at :44, it's out of place. It doesn't have enough of that evil feel.
The intro sounds like it would be awesome for a dubstep drop right at :44. The piano is very suiting. I love you you made the last notes stutter. It added a ton of ambience.
As far as the drums, they sound really cool, but they aren't really ambient. They make it more of a drum and bass. If you wanted to add a lot of reverb to them, and fade the volume, they would sound pretty cool for the ambience.
Overall this was a pretty good track. You accomplished most of the Halloween feel.