Why? Why did you make the greatest boss fight out of any module I have played or ran (I have played or ran pretty much everything to include available CCCs and Baldman Games), and put him into a disposeable story. It would have been so easy to build up this fight, foreshadow stuff in earlier adventures, just someone mentioning in passing, hey there is this headless horseman dude. But no, instead you create one of the most memorable characters and fights and make him barely relevent to the overall storyline except for the out of the blue thing that he has that the party needs.
If you get the chance to play this, I would definitely recommend it, but I would just note that it doesn't make a whole lot of sense within the overall season 4 storyline.