Hallow's End started yesterday and runs through October 31st. You can queue from the LFD menu.
The Headless Horseman drops 4 rings:
Wicked Witch's Signet
Band of the Petrified Pumpkin
Seal of Ghoulish Glee
The Horseman's Ring
There is no tank ring, plate dps, don't be surprised if the tank rolls against you.
The Horseman does not drop The Horseman's Sinister Slicer and The Horseman's Horrific Hood. They have a chance to show up in the Loot-filled Pumpkin. This is obtained on the first kill everyday.
If you're working on The Black Prince rep or leveling up, consider picking up Unburdened or Grim Visage buff for 10% more rep and exp per kill.