Here are my ideas:
A GM event: I think this would be a very good (and quick) idea. At a certain time of day on Halloween or it's eve, GM NPCs can hand out items or we could take the runescape approach and make rare drops.
Drops/Items:
Skeleton suit
Candy
Halloween basket (shield)
Skull Hat
Bedsheet
Black Desert SandStorm: Perhaps the most difficult idea I have come up with, but the desert's sandstorm would be temporarily black, and a cult could be summoning an ancient monster in the spider lair underneath the mine. (That place looks like a dungeon anyways.

Items/Drops:
Desert Scythe
Jeweled Cane
Skull Hat
Bedsheet
Halloween Basket
candy (if your that one unlucky guy)