The Headless Horseman is located inside the Graveyard of the Scarlet Monestary. This is a simple tank and spank fight. You'll essentially be killing the him three times before he actually dies.

Once you get him to 1% he will throw his head and dismount to chase it. You will hear him yell , "Get over here you idiot!" Everyone needs to move to the head now. His body will be immune to attacks and will occasionally Whirlwind. Stay away from these as it does a lot of damage! At the same time you are damaging his head, his body is healing. Once you damage his head to 60% or he heals back to 100%, he will rejoin and get back on his mount.

This phase is just like the first one, except he throws a pumpkin at a random player's head (never the tank). That person will take fire damage over time as well as become disoriented. Any abilities or trinkets that will remove control loss will work here. Once again when he reaches 1% he will dismount and chase his head. Once the head reaches 30% or his body heals back up he will rejoin.

During the third phase he will continue to do the same things, however he will not be throwing pumpkins onto people's heads. Instead he will toss four pumpkins on the ground that eventually come to life and attack the person with the highest threat. The tank should pull these as they hit rather hard. Have someone AOE the adds down and get the Horseman back down to 1%. When he separates his head will be at 30% so do your best to kill it this round or else when he rejoins he will continue to summon pumpkins.