The Dragonborn first came into existence thousands of years ago, after a warrior made a pact with an evil dragon, granting him power beyond his wildest dreams but leaving him mutated into an amalgamation of human and dragon. His bloodline also suffered the curse, becoming the first of the race that would come to be the Dragonborn.
Due to the curse, many see Dragonborn as unnatural and freaks, causing them to be essentially ostracized from many modern societies, forcing them out of their home of Drakonia and into small holdouts all over the world.