The Doom of Valyria was a volcanic cataclysm that took place approximately four hundred years before the War of the Five Kings. It destroyed the empire known as the Valyrian Freehold, shattered the Valyrian Peninsula and created the Smoking Sea. Its consequences were severe and long-lasting, extending to the present day.
The term refers to both the specific night when the Doom began and destroyed the Freehold, and to the ongoing volcanic eruptions and earthquakes that continue in the region to this very day.
Valyria was a city in southern Essos that became immensely powerful after its people discovered a way of taming and controlling the dragons they had found lairing in the nearby volcanoes known as the Fourteen Fires. Using the dragons as weapons of war, they threw down the then-dominant Ghiscari Empire and established a mighty nation of their own. According to tradition, Valyria's rise to power began five thousand years ago.
Valyria was not ruled by a single emperor or king, but by the council of Lords Freeholder, immensely powerful and wealthy landowners. Despite this apparent sign of civilization, the Valyrians also believed in slavery, buying and selling lives on the open markets. Some of these slaves toiled as miners underneath the Fourteen Fires, protected from being burned alive by Valyrian sorcery whilst they dug out precious substances.
On the night of the Doom, it is said that every one of the Fourteen Fires erupted in rapid sequence. Hundreds of smaller peaks, mountains and hills also erupted, along with channels of lava bursting through the ground. Earthquakes of staggering power shook the entire peninsula. The land itself cracked and started dropping into the sea. Even those cities spared immediate destruction - such as Valyria itself - were consumed by ash and soot lifted into the air by winds (hence the common description of the Doom "descending" on Valyria itself).
The mainland cities of Valyria, Tyria and Oros were destroyed in the Doom, along with hundreds of smaller settlements. The Doom triggered immense tsunamis which swept across the Gulf of Grief, Slaver's Bay and the Summer Sea. The Isle of Cedars was inundated, with the cities of Ghozai and Velos destroyed in the tidal waves. The devastation even reached to far Sothoryos, flooding the north coast of that continent. Fortuitously, the weather and the pattern of destruction meant that Volantis to the north-west was spared similar destruction.
The dragons of Valyria laired on the slopes of the Fourteen Fires. When the Doom took place, they were almost all killed. The only survivors were Balerion, Vhagar and Meraxes, the dragons belonging to House Targaryen, living in self-imposed exile on the island of Dragonstone far to the west.
The Summer Sea flooded the lands sunk by the Doom, reducing the heartland of the Valyrian Freehold to a cluster of shattered islands. The seas that surrounded Valyria became hot and gave off unusual smoke and smells, some of which proved fatal to breathe. The Smoking Sea, as it became known, was soon shunned by sailors from across Essos. A superstition grew which said that even setting eyes on the shores of Valyria would mean death. Despite this, some men (such as Gerion Lannister and Euron Greyjoy) would still brave the dangers to try to loot the cities of Valyria. Some have even returned.
At least some of the Fourteen Fires survived the Doom, with their surviving peaks still spewing lava intermittently into the sky. Those who have braved sailing close to Valyria confirm that the skies over the islands and the peninsula glow red at night from the active volcanoes. Because of this it is said that the Doom still holds sway over Valyria, although the frequence of eruptions and earthquakes is nowhere near as frequent as on that first night.
Politically, the Doom of Valyria destroyed the Valyrian empire. The colony of Volantis to the north-west proclaimed itself the new capital and tried to hold together the Freehold, but without any dragons was unable to do so. Other Valyrian cities to the north such as Mantarys, Elyria and Tolos also became independent. The other colonies in the west broke away, becoming the Free Cities. The Ghiscari were able to reassert themselves in the east. A few decades later, the nomadic tribesfolk of the grasslands to the north and north-east gathered together as the Dothraki and began raiding and enslaving surrounding lands. The period of one hundred years immediately following the Doom became known as the Century of Blood, as the various factions and powers of Westeros and Essos adjusted themselves to Valyria's sudden fall. This period ended in the defeat of Volantis's attempt to conquer the Free Cities and in the conquest of Westeros by Aegon Targaryen, the last dragonlord of Valyria.
The exact cause of the Doom is not known. It may have been a natural event, but some believe that gods of one type or another destroyed Valyria for its hubris. The Faceless Men, a sect of assassins now based in Braavos, claim that their founders were slaves working in the mines under the Fourteen Fires and had sworn to destroy Valyria, suggesting that they may have somehow caused the Doom. Whether this is true or a hyperbolic claim is unknown.