You can visit any Innkeeper once an hour to Trick or Treat. But the results are random - you don't know which you'll get - Trick or Treat:
For a brief period of time (less than a minute) you may be turned into a toad, a cat, or other familiars:
You may also be given a random costume which lasts an hour:
This is me as a wisp Hearthing:
I also like that you can dance in the same form as your costume:
You can ride a mount or fly while you're in most disguises:
Unless your disguise is a Shape Shift:
(Yeah I was not happy.)
Hopefully what you'll get is a Treat. Masks, costume wands or candy.
Here are some masks in action:
The candy as Doeg put it "the best food in the game." They restore 3% of Health and Mana per second for 25 seconds.