Once we were many, and walked in the sun...
Our people built mighty, sprawling cities with towers that stretched towards the black dome of the sky. We walked with our brothers and sisters, who worshipped the sun, while we revered his sister the moon and her children, the stars.
But we were betrayed.
The sun grew jealous of his sister's grace as she danced across the sky, each step different from the last. It was contemptuous of the stars, annoyed that such weak and frail lights should garner reverence rightfully owed it's might.
It whispered to our brothers and sisters, and they grew jealous of us in turn. One day, the sun tricked the moon into visiting him in his daylight sky, though the brilliance of his fire hurt her eyes. When she stepped close enough, he devoured her.
As we watched this celestial fratricide in horror, we found it mirrored in our realm as our brothers and sisters turned on us. They cursed us with their magics, blackening our skin and bleaching the color from our hair and turning our eyes as red as the sky Kestali died in so that they could know us by sight. They slew us where they found us, and drove us from the sun-touched surface, away from the light of our beloved stars, beyond the sight of our lady, the moon. Deep beneath the earth we fled from their anger, their hate.
Many were lost. We wandered the dark corridors of the earth until we found a place where the slain moon's blood had trickled through the ground to pool in the floor of a cavern that stretched almost as far and as high as the night sky had once done. As a final gift, she bestowed upon us the means of our salvation; the Silverweald, which grew from her shed life's blood and which nourishes us even now.
Our Lady speaks to us yet, for what is Death to an immortal? She bids us to patience, to peace. The way home is shut still, and will not open soon. We must wait, and gather our strength.
For once we were many, and walked in the sun. And so we shall be again.
((Note: This is the Underhollow's version of the myth. Different cities will have slightly different versions of this tale.))