Ten thousand years ago, a Native American priestess used all her mystical powers to create the first bronze alloy in the western hemisphere, which to her was the entire world. Subsequent metalworking would produce normal artifacts like you see in museums, but she poured everything she had and everything she knew into this first medallion, imbuing it with a power to transform people the way she transformed metallurgy. It could create all sorts of transformations, but, fortunately, once used it could not be used again for another half-cycle of the sun (i.e., 12 hours). Realizing the powerful effect of this medallion, the priestess kept the medallion a secret and, at the end of her life, gifted the medallion to a trusted young priestess, and made the girl promise never to reveal its existence to anyone for any reason, until she reached her own deathbed. The secrecy of the medallion was paramount.
Then somehow it ended up in the hands of a middle-aged astrology practitioner, who decided to let a friend borrow it before going on vacation. It would be more fun, you know?