Some random initial data: there are some "treat" encounters (gives out candy), and some "trick" encounters (where she turns your wine into poison or something), and occasionally a choice encounter where she lets you choose "trick" or "treat".

It seems the trick encounters show up if you have a Treat Effect (extends duration by 20?). You get 2 candy instead of 1 if you have a mask on for the treat encounters.

Looks to be 5 candies/effects: +2 power to each stat plus unopposed.

Also: no apparent result for wearing both treat baskets (weapon/offhand). Nothing special for full mummy outfit aside from the mask.