Just in case people haven't figured out how to get this: melee the Horseman, then stay alive until he dies. Can only get this once.
On that note, you can make two different things with it, the re-skin of the eyelander making it the Horseman's axe or you can get a random Halloween hat.
Head cutter axe: 2 refined + skullcutter + unusual haunted metal
Halloween hat: 4 refined + haunted metal.
Too bad you have to decide which one to craft since you only get one haunted metal.