The Rhoyne flows into the Summer Sea at the port city of Volantis, where it is several miles wide. It flows from the north-west to the south-east for several hundred miles, and forms the eastern boundary of the Disputed Lands. The Dothraki Sea lies to the east.
Millennia ago, the Rhoyne, as a vast river fertilising the lands around it, became the home of a people called the Rhoynar, who lived along its banks, eventually coming to build several city-states along its banks, including one called Anar. The Rhoynar were warriors who could be ruled by men or women.
One thousand years ago, six centuries before the Doom of Valyria, the armies of the Valyrian Freehold invaded the region surrounding the River Rhoyne. The city of Anar was destroyed by Valyria's dragons as a sign of might. Rather than submit, Queen Nymeria led a mass evacuation of her people. According to legend, ten thousand ships of all sizes were assembled, sailed down the Rhoyne and into the sea, and thence westwards to Dorne, in the south of Westeros. There the Rhoynar invaded and subjugated the land, aided by allies such as Mors Martell, whom Nymeria married and proclaimed the Prince of Dorne.
Following the mass Rhoynar evacuation, the Valyrians colonised the region, building immense roads linking the Rhoyne to distant Valyria, and constructing the city of Volantis at the mouth of the river as a port and colony.
After the Doom of Valyria, Volantis attempted to conquer the surrounding region during the free-for-all known as the Bleeding Years, and begged for help from House Targaryen, then controlling the island of Dragonstone in the Narrow Sea. The Targaryens refused, instead embarking on an invasion of Westeros.