People of Fallcrest, I implore you!
Bael Turath was the ancient empire from which the race of tieflings originated. It arose far to the south of the Nentir Vale region, and never exercised much influence over lands so far to the north. Bael Turath and Arkhosia, the empire of the dragonborn, were political rivals for much of their history.
Bael Turath rose to prominence as a human city-state more than 1,000 years before the rise of Nerath. The arcane power of the Turathi ruling classes remains the stuff of legend. It is a matter of some dispute how and why the members of these ruling classes became transformed into the race now known as tieflings. One popular myth is that the ruling class as a whole made a pact with devils, agreeing that in return for increased arcane power to fuel their endless wars against Arkhosia, they would take on the visible aspect of their fell benefactors. Another version of the myth states, by contrast, that it was a Turathi ruler centuries earlier who made the pact with the devils, binding his descendants to perform an unspecified service for them in the future. According to this version of the myth, the nobles of Bael Turath were cursed when they subsequently refused to honor the promise of their weak-willed forebear.
In any event, within a century of their transformation, the capital city of Bael Turath was destroyed in a great magical conflagration, believed by sages to have been the result of wartime magical experiments gone awry. The Turathi government collapsed, and the surviving tieflings, unwelcome now among human society, took up a less settled and more mercenary lifestyle.