He had 500,000 hitpoints on normal difficulty, more than 700 times the health of a raptor.
The Pumpkin King could attack with his swords. Nearby enemies were atacked by his feet, as well. He spawned more and more minions that also attacked players.
After a certain time he vanished in a cloud of smoke to do one of his special attacks:
- Moving in a line while spinning around, teleporting to a different position for a few times.
- Teleporting on a mountain top and shooting an burning red ball that exploded on impact.
- Teleporting on a mountain top and sending screamin meemies that follow players and attack them.
To get to the pumpkin arena, players needed to kill the Etheral Beast, a yellow glowing raptor. Killing him early could get you into the arena unprepared.
The incredible amount of health required players to bring in utility guns if they did not want to kill him with pistols. The arena had some elevated positions that were safe from most attacks. Reaching them was easy with the power jump augmentation. The weapon with the highest damage per second was the Gatling Gun.
When the first Spooktacular event ended, the patch notes said: "returns next year". (source)