The Dragon Medallion is an ancient artifact that can be used to control dragons of any species and bend them to the user's will. It has been lost and rediscovered many times over the ages, sometimes cropping up in the bedroom of a Chinese emperor or the ruins of a temple; some theorize it only appears to those worthy to wield its unfathomable power.
The Medallion was created by one of the ancient Dragon Clans of old. These clans dedicated their lives to studying, controlling and understanding dragons. The Dragon Clans existed around the time of the rise of the Chinese dynasties, living in secret around the world.
Dragon medallions were originally created by ancient, legendary Chinese warriors who were powerful and favored by the gods - the elite of the descendants of the dragon, as the Chinese people believed themselves to be.
The only dragons that the Dragon Medallion apparently cannot control are shadow dragons: nothing can control shadow dragons. It also seems to have no semblance of control over infinity dragons.