The Imperial Age was the time of two great empires – dragonborn Arkhosia and tiefling Bael Turath. Beginning early enough that no Nerathi history records recall a time when Arkhosia and Bael Turath did not exist, the Imperial Age ended a few hundred years before the rise of Nerath.
Bael Turath was founded by humans. When the empire expanded beyond the grasp of its power-hungry nobles, they swore to retain their rule even at the cost of their humanity. After destroying a few houses that refused, the nobles summoned devils and used rituals to seal their infernal pacts, becoming the first tieflings. Bael Turath gained the nickname “the Hell-Chained Empire” after this time.
The primary devil with whom Bael Turath entered into infernal pacts was Asmodeus, though rumor suggests Asmodeus compelled each the other lords of Hell to impart secrets of their magic to the warlocks of Bael Turath. The lords of Hell, loath to surrender an iota of their might to mortals, attached a price to their spells.
The tieflings of Bael Turath are responsible for the creation of several monster races, either by intent (such as the kruthiks, reptiles infused with fiendish blood) or by association with dark pacts (such as the skulks, a slave caste left out of the compact with Asmodeus, whose bloody rebellion earned them the boon of supernatural stealth from the demon lord Graz’zt).
Bael Turath and Arkhosia came into contact, descending into hundreds of years of warfare. Border skirmishes grew with each year, and several full-scale wars broke out, including the War of Wings, the War of Teeth, and the Wars of Ruin.
The Arkhosian capital, Io’vanthor, fell when it became corrupted by cultists of Tiamat, who opened to city gates to Turathi invaders, thinking the Turathi would slay the remaining worshipers of Bahamut and then they would slay the Turathi. The Turathi enslaved them all, sacked the city and abandoned it to sandstorms.