The Only Dragon Rider

Once, there were two peaceful people: dragons and humans. One day, the dragons wished to have more Moon Stone, a rare ressource. In exchange for it, the dragons would protect the human's land as written in the treaty.
But man's greed was once again their downfall.
The King wished to have a certain way to control the dragons and use them to conquer other lands through having dragon riders. Such a thing is forbidden by the treaty and no dragon will be ridden.
The King did not care.
The treaty was broken and the dragons leave.

Later,
At the base of the mountain which marks the beginning of dragon territory, there is a village. In said village, there is a poor orphaned girl. According to the other villagers, her mother was the village prostitute and her father was unknown.
They use that as an excuse to abuse the poor girl named Valeria. At the tender age of 8, she is running away from her abusers and she crosses the border of the dragons.
The dragons find her and instead of killing her, they keep her.
Valeria grows up with dragons, developing the ability to speak mind-to-mind with them. Dragons are magical beings, so being in their presence long has a tendency to make their magic rub off.
Their dragon King treats her as her own. Everyone loves her. She is even accorded the greatest honor of riding them. Valeria doesn't know that it is not normal to ride the dragons for she always had.

But one day, the Oracle Dragon has a vision. It is one of mass destruction and a darkness that will cover all the land. To prevent such a thing, the dragons must help the humans fight off the evil.
For Valeria, that means to return to her biological people. But she is now 18 and changed by the dragon's magic.