Progenitor Dragons

Before there was time, there were three dragons, Siberys, Eberron, and Khyber. Upon their awakening, they created - or discovered - the Draconic Prophecy. Perhaps because of this, they sought to create thirteen planes of existence, each embodying a concept. Their final work was the Material Plane, where all ideas would become manifest: a realm that could know war and peace, life and death, order and chaos. But cruel Khyber sought ultimate dominion over this new reality. She struck Siberys without warning and tore him apart. Eberron wrestled with Khyber and bound the traitor in her coils, but could not defeat her. So Eberron became a living prison, a world that would forever contain Khyber's evil.
Almost every culture of the world shares this story as a myth that explains the world. Shattered Siberys became the ring of dragonshards wrapped around the planet, said to be the source of magic. Eberron is the world, the source of all natural life. And Khyber is the Underdark and the source of aberrations and fiends, forever struggling against her bonds and yearning to destroy the world above.