The Grable Empire was the ancient empire that fell into ruin in the World of Atlas. The empire was overrun by Chaos the Beast and its minions. Jukal, the emperor, riding the great bull, Tanlavar, killed the Beast in a world shattering battle. To be victorious, Jukal had to turn himself into a horrible creature of malice. When the Beast was dead, this creature of malice remained and sought to destroy everything by killing the gargantuan turtle upon whose back the continent of the empire rested. The turtle sank to the ocean floor and the continent was bathed in a flood of impossible proportions. Only those that built arks survived. This happened at the dawn of the Age of Chaos. This Age lasted nearly 1000 years until the Beast was resurrected by the Drow and then defeated again by the divine spirit of Calgillien, the greatest general of the former Grable Empire.
As for the Grables, they were a people with runic blood. (Think mutants in the X-men for a very crude comparison to what a Grable was like). The runes in their blood gave them power over the elements, law, truth, fate, and even chaos. Chaos was fated to destroy the world, but Jukal was able to bend the prophecy to make it lower case chaos as opposed to Chaos the Beast,. That is why the period AFTER the Beast’s death was called the Age of Chaos. It was the time where lower case chaos worked to destroy the world and spread its evil throughout the lands and how the creatures of thought (man) tried to resist them. Some of the surviving men had Grable blood, but the ability to unlock its power was lost to most.
There was a place on the continent that tried to preserve the culture of the Grable Empire and worshipped its Immortal Council as gods. These were the Old Grable Kingdoms of Ranek, Haran, Strandi, and Polfany (Atlas holds up the continent on a mountain top in the southern part of Polfany.) That’s where the majority of the adventuring on the world has taken place in my home campaign. So, I’ve got the Old Grable Kingdoms covered.
Tanvarpov, the great ruin of the Grable Empire, is now the city of dragons. It was the richest city in the world and naturally was the most attractive place for dragons to land when the flood waters subsided (when Atlas lifted the land from the sea floor returned it to the surface.) Note: Atlas does hold up the earth, but its from fulcrum mountain and theirs some divine physics at hand where Atlas makes himself the center of gravity so the land actually falls up toward him. I wave my hands at exactly how that works, but the temple on Atlas Mountain contains no iron due to the powerful magnetic fields. Bronze weapons are thus considered holy to Atlas Worshippers.
As for the direct parallel to the Roman Empire, the Grellics were a people from islands south of the Oklossossian Sea. Some discontents left the Grellic City States to explore the north and the land washed away by flood. Some settled on the peninsula of Vescaly and formed new city-states. The greatest of which was Latto. Latto conquered the other city-states on the peninsula to create an empire. That empire crumbled from within before it expanded beyond the peninsula, primarily due to a lack of gods. They tried to worship the Grellic gods, but few priests immigrated with them so they had ill-trained amateurs trying to recreate complicated rituals. The result was powerless prayer. They learned of the Grable Gods from the north, but only took that the emperor and his court were gods, so they decided to make themselves gods, too. The emperor was named Jupiter and the governors were all named Mars, the gods of war.
After the Great War at the end of the Age of Chaos, Demogorgon led the remnants of his defeated army to the island now called Abyssus and conquered the city-state there. The Latton Empire may reform in the joint efforts to remove him from that seat of power.