Balance of Those Lost (the dragon prince x non-binary reader)

Long ago, Xadia was one land rich in magic and wonder. In the old times, there were only the six Primal Sources of magic. The Sun. The Moon. The Stars. The Earth. The Sky. And the Ocean.
But a thousand years ago, a human mage discovered new magic - the seventh source.
Dark Magic. It used the essence within magical creatures themselves to unleash dark power.
Horrified by what they saw, the elves and the dragons put a stop to the madness. They drove every last human to the west. And so, the continent was divided in two. In the east, the magical lands of Xadia. And in the west, the human kingdoms. For centuries, the King of the Dragons himself defended the border.
The humans called him Thunder, for when he spoke, his voice shook the earth and the sky. But on the eve of last Winter's Turn, the humans used unspeakable Dark Magic to slay Thunder.
Then, without mercy, they destroyed his only egg, his heir the Dragon Prince, and killed the dragon spirit, a being that was both dragon and elf with connections to the six sources. Now the world stands on the edge of all-out war.