From WoWWiki
The Savior of Hallow's End is related to the seasonal world event Hallow's End. Completing this achievement is criteria for the meta achievement 
[Hallowed Be Thy Name].
Locations
The world spawns of the Headless Horseman are actually the 
[11] Shade of the Horseman. Every few minutes during Hallow's End, the Shade will attack 
Kharanos, Goldshire, Azure Watch, 
Razor Hill, Brill, and Falconwing Square. These quests cannot be soloed - they require at least a small handful of organized players.
External links
The Savior of Hallow's End
[Hallowed Be Thy Name].
