The Valyrian Freehold was a great empire of the eastern continent, of which it conquered vast swathes over a period of several millennia before it was destroyed in a cataclysmic event known as the Doom of Valyria. In the aftermath of its fall, one Valyrian noble family, the Targaryens, launched a successful invasion and conquest of the continent of Westeros.
Five thousand years ago, the Valyrians were a people of Essos, living on a vast peninsula extending southwards into the Summer Sea, west of Slaver's Bay and the great Ghiscari Empire which dominated the eastern side of the bay. The Valyrians appear to have been shepherds and pastoral folk, not worth the bother of conquest. The Ghiscari claim that their empire was at its height when the Valyrians were still sheep-herders.
At an unknown time, the Valyrians discovered dragons lairing in the Fourteen Fires, a circle of volcanoes extending across the neck of the Valyrian Peninsula. They discovered a means of harnessing and controlling these creatures, and using them in battle were able to repel Ghiscari invasions and then embark on a campaign of conquest. Ghis launched five separate invasions, wars and rebellions against the Valyrians before they were finally cowed. The Valyrians forged a massive empire stretching across much of southern Essos over the next several millennia.
By approximately one thousand years ago, the Valyrians had expanded into the western coastal regions of Essos, the area bordering the Narrow Sea, and founded the city of Volantis as a colony-state at the mouth of the great River Rhoyne. The people of the region, the Rhoynar, went to war against the Valyrians but were defeated. Nymeria, warrior-queen of the Rhoynar, led an evacuation by ship to Westeros, where they invaded and subjugated Dorne.
The Valyrians continued to colonise the western coastal regions of the continent. Two centuries after the flight of the Roynar, a religious sect, the Moonsingers, led several thousand refugees northwards, seeking sanctuary, until they found a lagoon sealed off from the outside world by mists and reefs. Here they founded the Secret City of Braavos, and endured in secret for four centuries.
In the meantime the Valyrians founded or conquered the cities of Myr, Lys, Lorath, Tyrosh, Pentos, Qohor and Norvos in addition to Volantis. They also established a fortress on an island in the Narrow Sea called Dragonstone, and used it as a trading port with the continent of Westeros to the west.
Five centuries ago, House Targaryen, a noble house of Valyria, elected to take up the governance of Dragonstone. According to some histories and rumours, the Targaryens had the gift of foresight and had elected to leave Valyria on the basis of a prophecy that the Freehold was doomed.
A century after the Targaryens arrived on Dragonstone, the Doom fell upon Valyria. The precise nature of the Doom is unclear, but the Valyrian Peninsula was shattered and torn away from the mainland along the side of the Fourteen Fires, whilst fire and ash descended across the peninsular. For this reason the Doom appears to have involved the eruption of the volcanoes in a titanic blast, with fire and ash falling hundreds, if not thousands of miles away. The homeland of the Valyrian Freehold was utterly destroyed in this disaster, and the colony-states immediately started fighting for dominance or independence, triggering a period known as the Bleeding Years.
The destruction of the Valyrian Freehold freed many vassal peoples from their control. The Ghiscari rapidly regained control of their own lands, although they failed to reestablish a strong empire. The western colonies broke away and became the Free Cities, despite attempts by Volantis to reforge them into a new Valyrian empire. A century after the Doom, the Targaryens embarked on an invasion of Westeros under Aegon the Conqueror that delivered the entire continent to their control. Valyria became a feared, shunned land where demons and ghosts were said to hold reign. The elimination of the Valyrians permitted the Dothraki horse-riders of the central steppes to grow in number and achieve dominance of the central regions of the continent.
Some Valyrian towns and cities remain extant, as do their great roads which criss-cross much of the coastal regions of Essos, stretching for thousands of miles from Pentos to Slaver's Bay.
Valyria was not a traditional kingdom or empire, but was controlled by a council of 'Lords Freeholder', nobles who controlled vast areas of land (freeholdings). The Freehold was divided into colonies and vassal states, although the exact mechanisms of control and authority remain unclear.
Valyria had a large army, but its primary military asset were its dragons and pyromancers. The role of dragons in the Valyrian army is unknown, but theorised to involve scouting, shock assaults from the air and possible transportation, as well as being able to relay messages at speed. It is unknown if Valyria had a powerful navy.
It is known that the Valyrians defeated Ghis in five separate wars, as well as conquering the Rhoyne region in a military campaign.