East of Westeros lies the Smoking Sea, where no ship dare sail. There are those who swear it to be demon-haunted, and who’s to say they’re wrong? For it was there, thousands of years ago, that a cataclysmic event occurred, destroying one of the great civilizations in history.
The precise details of their origins are lost to us, but it is said the Valyrians were once a modest community of shepherds, tending their flocks on a small peninsula of the great eastern continent. One fateful day, in a volcanic area known as the Fourteen Fires, they made a shattering discovery: dragons.
They were monstrous, scaled creatures, with massive wings, sharp claws and fiery breath. They were also said to have a deep-rooted connection to magic.
In time, the Valyrians were able to tame the beasts. Harnessing their immense power, they established a city of wonder unlike any before or since. They became skilled at sorcery and metallurgy, creating uncommon weapons of spell-forged steel. Wielding these weapons, astride their dragons, the Valyrians conquered the surrounding lands and slowly expanded west.
At the time, the Ghiscari Empire dominated much of the great eastern continent, and tried to stop Valyria’s expansion. The Ghiscari legions attacked the Valyrians five times, but could never defeat them. Finally, the Valyrians marched on their capital - Old Ghis - and obliterated it, turning its streets and buildings to ash with dragon flame, and wiping the Ghiscari people and their culture off the face of the earth.
The Freehold of Valyria, as it came to be known, became the most advanced civilization in the known world, with its own language, gods and culture. The Valyrians' reach extended far and wide, covering most of the continent. Great cities were built and roadways paved, all of which led back to Valyria. The Freehold would prosper for nearly five thousand years. But it was not to last.
An event that became known only as the Doom laid waste to the Valyrians, their capital city and its surrounding lands. The peninsula itself was shattered, becoming what is now the Smoking Sea. Every dragon was thought to be lost, as were the Valyrians' spells, knowledge and recorded history. Thus the mighty empire collapsed.
What caused this cataclysm? No one knows for certain. Some say it was a volcanic eruption. Others say the Valyrians’ own sorcery got the better of them. In any event, the Doom’s devastation of the Valyrian people was total… with the exception of a small rocky island in the Narrow Sea: Dragonstone. For it was there that the Targaryens - the last of Old Valyria - dwelled. They would lie in wait for another hundred years before unleashing the fury of the dragon on another continent - Westeros.
Source:
http://gameofthrones.wikia.com