To start you need to get a Blue Blanket from the corpse of a Pumpkin Follower. The corpse is usually found near spawns of Pumpkin Lords on the landscape during the Harvest festival.
Take the blanket and give it to the jack o'lantern statue past the Locked Door in the east hallway of Colosseum to be portaled to the Arena of the Pumpkin King.
Note: The blanket flags you to use the statue for 20 hours and is used up when handing it in. You will need more blankets if you wish to enter after this time.
You can fight the pumpkin without completing the Colosseum. What you need to do is buy one egg and give it to the Arbiter in the main room. That will flag you to use the locked door in the hallway past Diemos.
Make sure everyone has a blanket, otherwise they can't enter the arena.
Kill the king and loot the Pumpkin King token from his corpse and turn it into the Master Arbitrator. You get will receive xp, a title and the Pumpkin Shield as a reward.
You give Arena of the Pumpkin King Blue Blanket.
The statue's eyes flash and you feel the pull of portal space.
You give Master Arbitrator Pumpkin King Token.
Master Arbitrator tells you, "I see that you have proven yourself against the Pumpkin King. Well done."
Master Arbitrator tells you, "Since you have slain him, you must be will be known as the "Pumpkin Throne Usurper.""
Master Arbitrator tells you, "Oh, we found this mixed up in the mess you left in his arena."
Master Arbitrator gives you Pumpkin Shield.
Master Arbitrator tells you, "And I believe that you have earned a bit of practical knowledge."
The Arbitrator reaches out and lightly touches your forehead, sending a jolt of new understanding through you mind.
You've earned 248,000,000 experience.