Son of the [Great Hero] Ruskatia and descendant of a fallen god, [Mountain God], Satori.
Ruskatia became pregnant due to a spurt of the Satori's blood. Though it's not certain why she gave birth to Rorgis, her sworn enemy's child, but the circumstances caused the two to become one.
However, Ruskatia had lost the ability to fight on her own, so instead she trained Rorgis so that he could fight on his own. This is the story that was carved into a lithograph that's passed down among the royal family. Well, that's the long and short of it, anyway.