The Doom of Valyria was a catastrophic event that took place nearly four hundred years before the events of Game of Thrones. It caused the fall of the Valyrian Freehold and would lead, indirectly, to the conquest of the Seven Kingdoms of Westeros by House Targaryen.
For nearly five thousand years, the Valyrian Freehold had remained the dominant power in the continent of Essos, thanks to the dragons, mighty beasts that were key to their power. However, one fateful day four hundred years ago, a cataclysm - which became known in later years simply as "the Doom" - laid waste to the Valyrian capital, its people, and the surrounding lands. The string of volcanoes near Valyria known as the Fourteen Fires exploded in a massive eruption, consuming the city of Valyria in ash and flame, and the entire region was wracked by corresponding earthquakes. The Valyrian peninsula itself was shattered and the ocean came sweeping in, becoming the area now known as the Smoking Sea - which no sailor dares cross and which is said to be haunted by demons.
The exact cause of the Doom remains unknown: some believe the volcanic eruption was a natural disaster, while others believe it was inadvertently triggered by the Valyrians' own sorcery. Whatever the cause, Valyria was destroyed, and all their dragons were lost with it, along with Valyrian recorded history, spells, and knowledge. With its center of power destroyed, the Freehold collapsed. Of the mighty families of dragonlords who ruled the Freehold, only one survived, House Targaryen, which had established on Dragonstone years before the Doom. Most importantly, the Targaryens on Dragonstone possessed the only Valyrian dragons that survived the Doom, the only remaining dragons in the known world.
In the books
In the A Song of Ice and Fire novels, the Doom of Valyria was an epochal event which dramatically changed the course of world history. With the heartland of the Valyrian realm destroyed, its surviving colonies to the west and east re-asserted their independence, politically fracturing the continent and initiating a series of savage wars for dominance known as the "Bleeding Years", which lasted for a century. Several of these colonies formed into the Free Cities in the west, and the cities of Slaver's Bay in the east. Meanwhile, without the Valyrians and their dragons to keep them in check, the Dothraki mounted hordes swept across the continent from the central plains of the Dothraki Sea, plundering and carving out new kingdoms from surrounding areas - from the Dothraki point of view, the Doom started their own golden age of conquest and expansion. In the extreme northwest of Essos, the hidden city of Braavos, founded by escaped slaves who fled Valyria, now revealed itself to the world. While the other Valyrian colonies were left in chaos, Braavos was an island of stability because it had never relied on political and economic ties with the Freehold, and it quickly rose to become the most powerful of the Free Cities.
Massive tidal waves caused by the eruptions and earthquakes resulted in giant tsunamis which struck many far away coasts. The three major cities of Slaver's Bay were shielded from the tsunamis by the Isle of Cedars, a large island between the bay and Valyria; however cities on the Isle of Cedars were swept away by the waves, and the island has been uninhabited ever since. The continent of Sothoryos to the south, however, was not so lucky: there were no major barrier islands to shield it, and its northern coast bore the full brunt of the massive tidal waves. The flourishing kingdoms on the north coast were devastated, and Sothoryos' once-great cities of Zamettar and Yeen were left in ruins. The peoples of northern Sothoryos have not been able to rebuild to their former levels of power in the past four hundred years, making them easy prey for slaving raids from Slaver's Bay to the north.
The Doom is partially inspired by ancient disasters such as the destruction of Pompeii by the volcanic eruption of Mount Vesuvius; author George R.R. Martin has said that the Valyrian Freehold itself is loosely analogous to the Roman Republic in his fantasy world. It is also partially inspired by myths about the fall of Atlantis.
- Doom of Valyria at A Wiki of Ice and Fire (spoilers from the books)