Like Stealing Candy...

From the introduction:

Introduction
The Midnight Star Ruby is one of Vladamir Manor’s most prized possessions, but when a rival band of thieves tries to smuggle it out of the manor on Halloween night, the Chameleons must step up to the challenge and intercept it for themselves before the precious gem is lost!
As a Chameleon, you must infiltrate the manor and intercept the ruby. Avoid letting on that you’re the member of a secret thieves guild and remember, never ever mention the mark’s name aloud or give away the name of your secret hideout!