A thousand years ago, a meteor came. It carved a path through the middle of the super-continent, almost bisecting it in half (for reasons yet to be discovered or searched for, the impact didn't do more, like end all life).
What remains connecting the two halves of the continent is a small strip of land to the north; mostly glassy-desert, with not much hope for plant growth in the coming centuries. Seeing the potential for setting up tolls, the dragons (and dragonborn) moved to the strip, charging a toll for passage. It came to be known as the Dragon Strip.
Over time, however, it was discovered that the impact point of the meteor, to the south of the strip, caused a sort of rift between planes/dimensions/realities, allowing monsters of all kinds to enter through. The dragons vacated the strip, and established a walled city on either end.
The cities are connected to each other by a magi-tech adamantite train with a crystal powered forcefield. Anything short of that wouldn't last against hordes of death dogs, and ravenous purple worms. The cost was more than made up for by the extra toll for using the train and the revenue that came from markets set up in either city.
Several generations ago, the dragons exiled their white siblings to the desert. The cause isn't known, even by some dragonborn.
The whites, already primal by nature, thrived(in a sense) in the desert. They established several settlements around the desert. Their settlements don't get attacked by purple worms for some reason.
Some people don't wish to pay for the exorbitant train tickets. Others (very rare) don't have a pleasant train ride, either by malfunction, determined monster attack, or mysterious circumstances, and fall of the train. Both kinds find themselves helpless in the desert, surrounded by monsters, or nearly starving/dying of thirst.
Some very lucky ones are found by wandering white dragonborn, and are taken to their settlements.
Do you guys want more?