Let us begin our grand adventure with something simple, yet complicated. I have created a sort of headcanon revolving around the disappearance of the dragons on July 7th, year 777.
I believe that the dragons disappeared into the bodies of people seemingly chosen by fate. The person who was to receive the dragon soul -coexisting with their own soul- traumatically passed away on their eighteenth birthday. Each representative died in a way that slightly mirrored the dragon they were chosen to harbor. The technical name for these people is: DragonBorn; born again by the dragon soul.
Each of the selected individuals coincidentally turned eighteen upon July 7th, 777. They were all born with heterochromia -meaning the person had two different colored eyes- but some were more noticeable than others. Another factor that sets these dragon harborers apart from the "regular" mages are the swirled designs imprinted on the forearms, with the name of the element on the back of each of their hands.
They are relatively powerful people, usually being kept in captivity until they die (again). Unfortunately, they are immortal -unable to be killed by natural causes- upon gaining the dragon soul.