Just a basic guide, i knew when it came out, i was clueless, heres just some facts.
To get the quest you must visit the pumpkin in the back of SM Graveyard, pretty easy to see. It counts as a daily quest, and everyone can do it once per day.
Fighting HHM (Headless Horsemen)
Stage 1:
Easy, just tank and spank, whoever summoned him will get aggro, but just a quick tank or growl will help. He has cleave, so non-casters do it from behind. Once he reaches 1% his head will fly off, quickly find it and DPS it. He will wander around pointlessly
Stage 2:
Same as before, let tank regain aggro, but this time he will randomly conflagration someone, similar to Drak event, they wander around for a few seconds and they damage nearby allies. From experience its usually someone that isn't a healer. When he loses his head he follows it whirlwinding around attacking nearby enemies.
Stage 3:
Loses the ability to throw fire, but throughs down pumpkin plants, which have around 10K hp and hatch around 15-20 seconds and attack. Good time for a pally to lay down consecration. Just full DPS down the Horsemen and ignore the pumpkins. When he eventually loses his head, DPS the head down, as soon as it dies all pumpkins will disapeer.
You get various brooms worth good gold.