Golden tablets with the seal of the ruler that issued the pass and the seal and name of who it was issued to. About the size of your palm of solid intricately etched gold. The holder of the tablet is backed by the authority of the king to make any required purchases on credit or enter any place they deem necessary. A very powerful token indeed.
If you want to add a "supernatural" touch to your campaign, define the rules of magic and the universe. Make your players comfortable with those rules. Then your new supernatural creatures must then break those rules.