The Ashen Covenant Rises
Bael-Turath was the largest human empire in the history of the world.
Their noble families were intensely paranoid about loosing their powers and so potent was this obsession that the Devils of the Nine Hells felt their desires.
The Fiends sent visions and dreams to the heads of the noble houses that gave them the complex and horrid directions to a month-long ritual that would give them great power. The details of that incantation are long forgotten but scholars note that it required blood sacrifices of innocents and signing over many souls to the devils.
The Turathi sacrifices were rewarded and after that long month all of the nobles underwent a painful transformation. They grew leathery hides, horns, fangs, claws, and tails. They became masters of infernal fires and they suddenly understood dark magical abilities. They became the first Tieflings.
They ruled the empire with and iron fist and took slaves from all the neighboring nations. Eventually they grew so powerful that they sought a way to remove the Devil’s influence over them so that they would truly have no masters above them. Seeing this insult, the Nine Hells took back much of their strengths that had been loaned to the Turathi empire. In the midst of a war with Arkhosia, kingdom of dragonborn they were defeated and their empire crumbled. The only vestige of their nation is a city-state across the sea to the east called Vor Rukoth. There the tieflings and lesser devil tyrants lord over subjects as if their empire had never fallen.