Valyria was a great city located on the continent of Essos. It was destroyed in a volcanic cataclysm known as the Doom of Valyria, which took place approximately one hundred years before Aegon I Targaryen invaded Westeros, and four hundred years prior to the War of the Five Kings. Valyria was the capital city of the Valyrian Freehold, one of the most powerful empires in the history of the known world.
Valyria was founded more than five thousand years ago. Shepherds found dragons lairing on the slopes of the Fourteen Fires, a chain of massive, active volcanoes stretching across the neck of the Valyrian Peninsula. By means unknown, but possibly involving magic, the dragons were tamed and the Valyrians were able to use them for transportation and war. The Valyrians threw down the then-dominant Ghiscari Empire and conquered all the lands surrounding Slaver's Bay and the Gulf of Grief. The city of Valyria grew from a backwater to the greatest city in the world, the center of a vast network of straight roads extending across much of western and southern Essos. The Lords Freeholder met in the city to debate policy and rule their vast lands.
The city of Valyria was not directly destroyed by a volcanic eruption during the Doom, as it lay more than a hundred miles south of the Fourteen Fires. However, a colossal amount of ash was propelled far from the eruptions and descended on the city, smothering it (hence the common phrase that the Doom "descended" on Valyria). How quickly the city succumbed and how many people were able to escape is not known.
Valyria today survives as ruins, located on the largest surviving fragment of the shattered peninsula. By all reports, the city is at least partially intact, preserved by the very ash that destroyed it. Getting to the ruins is highly problematic, as the Smoking Sea that now surrounds the island gives off poisonous fumes that can quickly kill a person and there are no bridges or other links to the mainland to the north. In addition, there is a strong superstition in Essos that to sail within sight of the shores of Valyria means death, which discourages travel there. Gerion Lannister is one of the more notable travelers to have made the long journey to Valyria and not returned.
However, a significant number of looters have reached the city safely over the years, enough to pick the city clean of many significant treasures and artifacts. Euron Greyjoy is known to have sailed to Valyria and returned safely with an ancient dragon horn.