This was very good! I think you nailed a lot of things that make a good boss theme while managing to capture the Halloween theme. The orchestration feels well done and very fitting. There is a nice balance with the accented rhythm and faster-moving parts. I especially love the brass transition at 00:30. I got some Hollow Knight vibes.
I think my only feedback/criticism is the length/development of the piece I wished for more. I think the material you presented during the duration of the piece has the potential to expand on. This is something that I can struggle with but I also think if you managed to do that you would reach a new level. But not every boss fight requires a 2+ min theme.
This is already super good to the extent that I craved for more.
Love this one!