The Headless Horseman was an NPC based on the American folktale. He would spawn in the Halloween Mega Base and would give "Halloween Quest".
The Halloween Quest is started after you speak to the Headless Horseman. He will give you a quest to collect 5 Space Pumpkins and put them in your Warehouse. To obtain a Space Pumpkin, you would have to defeat the Grim Reaper, which would spawn in servers. Once the Grim Reaper is destroyed, it would drop 0-4 Space Pumpkins.
Fighting the Grim Reaper was not easy, as it was very powerful and could solo multiple Dreadnoughts. However, some skilled pilots killed the Grim Reaper without help. The Grim Reaper's map icon was similar to the Alien Punisher.
After you complete the quest, you would be rewarded with the Reaper.