The subject of this article or section is part of Hallow's End, a seasonal event that lasts two weeks. Once the event has run its course, this will no longer be available until next year, but there are no guarantees.
The Horseman was once a knight of the Silver Hand, but is now cursed. He is of the belief that he is alive, while those who face him are dead. When defeated, he is revealed as Thomas Thomson (aka Sir Thomas).
After using the Dungeon Finder and entering the graveyard, click on the Pumpkin Shrine to summon him. Each character can summon the horseman once per day.
As of Mists of Pandaria, players need to be a minimum of 89 to queue for the dungeon. The Headless Horseman can no longer be reached without using Dungeon Finder as he appears in a special instance seaparate from the regular versions of the Scarlet Monastery instances.
Whirlwind: After losing its head, the body will AoE whirlwind.
Conflagration: An AoE DoT, can only be removed with a PvP Trinket or other similar abilities(Ice Block, Divine Shield, etc). Ticks for a percentage of your maximum health every second and confuses the target. Lasts ~5 seconds. Feigning Death right before the conflag hits will cancel the fear effect, although the emotes and animations will still proc. You can attack/move as normal. Not sure if this also cleanses the DoT too.
Pulsing Pumpkin: Tossed on the ground during phase three. Casts sprout after ~15 seconds. If a pumpkin lands on a player, they get a disease named Squash Soul which reduces Spirit for 20 minutes.
The Headless Horseman encounter is a three phase fight.
Phase one
The Horseman makes his entrance and engages whoever lit the candle. The tank should taunt immediately if they did not start the event. Watch out for Cleave and DPS him down.
At 1% the horseman will go immune, toss his head, walk around disoriented, and occasionally use a Whirlwind attack. The Head of the Horseman will become an active target while the body heals to 100%. All DPS should have a target macro for Head of the Horseman. After the party damages the head to about 60%, or the body heals to 100%, the head will rejoin the body and the next phase begins.
Phase two
When the Horseman rejoins with his head, he targets the player with the highest threat. This is the same as phase one except he will randomly cast Conflagration on a random party member (not the tank). You can use a PvP Trinket to remove the Conflagration. Healers and ranged DPS should be alert to the location of both tank and Conflagration target; steer clear of these.
Phase two ends if the head reaches about 30% health, or when the body heals to 100%.
Phase three
The Horseman does not use Conflagration in Phase Three. Eventually he will toss four Pulsing Pumpkins, which sprout Pumpkin Fiends after several seconds, and they will completely heal. The pumpkins have around 10k health and the Fiends will crit cloth wearers for 1k. The fastest course of action seems to be having the tank stand in the middle of the pumpkins with focus abilities at the ready. The rest of the party continues DPS on the Horseman until his head is tossed. If the party has low DPS, a second or even third wave of Pulsing Pumpkins can be spawned before the head is tossed.
Tanking the Horseman on the steps to the underground area will cause all the spawns to land very close to the boss. This should make it very easy for the tank to pick them all up.
Once the Head of the Horseman dies, the Headless Horseman has been defeated, and the remaining pumpkins will die. If you don't kill the Head of the Horseman before the Horseman heals to 100%, Phase Three will repeat, meaning you have to nuke the Horseman again to get him to toss his head.
Phase 3 notes
Having a Paladin or Death Knight tank makes this Phase much easier due to Consecration or Death and Decay. The rest of the party hugs the tank, inside the spell's range, allowing the tank to pick up the Fiends, thereafter the DPS can burn these down.
The key to the Headless Horseman is to find and DPS down the head as fast as possible when it is off the Horseman. The head does no damage so all DPS should feel free to unload on it. The macro "/target Head of" helps with this, as it allows you to pick up the head almost as soon as it detaches. To make certain that the macro does not inadvertently target the Headless Horseman, you can use "/targetexact Head of the Horseman".
Quotes
On Start
(Player) says: Horseman rise...
(Player) says: Your time is nigh...
(Player) says: You felt death once...
(Player) says: Now, know demise!
Headless Horseman: It is over, your search is done. Let fate choose now, the righteous one.[emits a horrifying guffaw]
Lost Head
(Head of the Horseman) Get over here, you idiot!
Rejoined
Here's my body, fit and pure! Now, your blackened souls I'll cure!
Conflagration
Harken, cur! Tis you I spurn! Now feel... the burn!
Sprouting Pumpkins
Soldiers arise, stand and fight! Bring victory at last to this fallen knight!
Horseman Death
This end have I reached before. What new adventure lies in store?
Player Death
Your body lies beaten, battered and broken. Let my curse be your own, fate has spoken.
Equip: Increases your critical strike rating by 357.
In addition, everyone in the party gets 20 [Tricky Treats] after every kill. You will also receive a [Loot-Filled Pumpkin] the first time you kill him each day.
Pre-Mists loot
This section concerns content exclusive to Cataclysm.
For historical purposes, these items were only available prior to Mists of Pandaria.
When the Horseman was introduced in 2007, making multiple attempts required resetting the instance each time and creating multiple instance IDs, running up against the 5 instances per hour rule. In 2008, the event was changed so that resetting the instance was no longer required.
In Hallow's End 2010, there was a bug in Patch 4.0.1 which would cause the game to freeze upon hovering the mouse over the pumpkin. A similar case was occurring in Ulduar.
Media
Videos
Past
Prot Pally Solo
Headless Horseman Kill Paladin Tank
Headless Horseman Kill Mage
Rogue Demonstration
Patch changes
Patch 5.0.4 (28 August 2012): Level, loot updated for level 90 characters.
/Patch 4.0.1 (12-Oct-2010): Level, loot updated for level 85 characters.
Patch 3.2.2 (2009-09-22): "I posted this a while back, but the Headless Horseman is indeed level 80." (Source)