The Last Days
According to myth, Khyber is the name of one of the three first dragons, along with Eberron and Siberys, who ruled all. They discovered, or created, the Draconic Prophecy, and its words brought reality into being.
Thirsty for blood after his defeat of Siberys, Khyber wheeled on Eberron. The bloodless battle, a fierce dance, continued for eons, neither dragon gaining ascendancy over the other. At last Khyber grew tired, and Eberron enfolded and imprisoned Khyber in her own body.
Both dragons slumbered after their long warring and hardened into earth. And so the world was born, Eberron forming its surface and Khyber its dark depths.
Passages into the deepest parts of Khyber open into the Chaos of Khyber, also known as the Elemental Chaos. Full of destruction and danger, this is the birthplace of the fiendish overlords of The Age of Demons. While there are a few areas within the Chaos that hold a certain permanence, the rest is as the name implies: chaotic and random.
The influence of the Elemental Chaos is felt most strongly in the subterranean realm of Khyber, but it occasionally reaches as far as the surface of Eberron.