Nice piece, I like how driving the bass is! Also, each section works well together.
For me I do think I'd prefer a bit more "punch" to the mix, maybe playing around with the EQ or maybe distortion to get a more gritty sound, but then maybe the bass wouldn't be as prominent... I think the theme works well as a boss theme but is not particularly Halloween-like. Halloween doesn't necessarily have to mean dark, doing a more fun/playful Halloween variation could work.
I also really like your sketch and that you had a vision that fitted the piece. Overall solid ideas though not fit for the Halloween theme. Still sounds fun though!