Paollo explains it's powers to Credenza.

The Soul Key, also known as The Necklace of Dragonfly is a powerful magical object given to Credenza from Prince Paollo Majestan to aid her in the fight against The Great Raven and finding the Heirs. 

The Soul Key gave numerous powers. Not only did it open Credenza's magic potential, but it could also give someone a human soul, as shown with Raven.