The Mask Task

The Mask Task is related to the seasonal world event Hallow's End. If you destroy the mask you have received, the criteria stay completed. Note that masks are BoP and cannot be traded.

Source
All the masks can be purchased from just outside Stormwind City and  in the Ruins of Lordaeron for. Some of them can be found within Handful of Treats and the Loot-Filled Pumpkin.