Scales of Eberron
Khyber is said to be the body of one of the three Progenitor Dragons that created the world of Eberron. It is a massive subterranean realm that is filled with Devils, Deemons, Daelkyr, nightmarish, and the sleeping Rajah that were bound by the Dragons and the Gatekeepers.
One of the regions of Khyber is the Vault of the Keeper, where souls taken by Keeper’s Fangs are delivered. There, an ancient Draconic Lich claiming to be The Keeper reigns over them and inflicts hellish suffering upon them.
When something goes wrong in Breland commoners often mutter the curse “By Khyber”, or if they wish to swear at someone they might say “Khyber take you!”.