High Kingdom of Ocania

The High Kingdom of Ocania, also simply known as High Ocania, was a powerful realm which existed in the region that is now known as the Duchies, in the north-east of Var Venail. At its peak, it had a population of 23 million, making it the most populous nation in the world at the time. It was largely populated by humans and eastern red elves. For its eight centuries of continued existence, the High Kingdom governance experienced substantial transformations. It initially was a feudal monarchy which eventually became a centralised absolute monarchy, and during its last 180 years was converted into a confederation. Its capital was the great city of Gamachaer, on the coast of the Inner Sea.
The High Kingdom broke away from the Old Empire during the Third Great Uprising. This makes it the oldest of the human empires. It was first a union of the local warlords which fought the elves, but within a generation became a feudal kingdom. Over time it centralised and became an absolute monarchy. For over 500 years Ocania existed in a state of near-perpetual internal peace. Starting in the late second century of the New Age, the High Kingdom began a steady decline. It was ultimately destroyed in when it was invaded by the Hosaq Empire. 
In sharp contrast to the Old Empire, the High Kingdom left a lasting legacy. Its administrative system of provinces would later be copied by many larger states. Its architectural and technical achievements, such as its aqueducts and roads have been preserved through time and remain some of the largest construction projects in the world. The period of the High Kingdom is considered by most to be a golden age of peace and prosperity, both among scholars and rulers. Ocania's main tradeport, Sargot, eventually grew to become the largest city in the world.