The Doom of Valyria refers to a cataclysmic event that occurred approximately 400 years ago, causing the downfall of the Valyrian Freehold and the destruction of Old Valyria. The cause of The Doom is unknown. Mountains exploded, Valryian volcanoes shot molten rock to the sky and the earth opened up to swallow entire towns. In just one day, most of Old Valyria was destroyed and sank below the sea, bringing end to one of the greatest civilizations the world had ever known.
As a result of The Doom, much wisdom and magic of ancient Valyria was lost, such as the knowledge of how to forge Dragonglass. Today, there is little left of Valyria, except for some ancient ruins. It is said that these ruins are rich with valuables that survived the Doom, such as Dragonglass and Valyrian Steel; it is also said that these ruins are forever haunted. Many men have sailed to the Valyrian ruins in search of old Valyrian treasure, but no man has ever returned.
As the capitol of the Valyrian Freehold no longer existed, each individual Freehold sought their own independence, causing much war and bloodshed between cities over the next 100 years. Eventually, the nine Free Cities were established as we know them today.
While all other noble Valyrian families perished in The Doom, House Targaryen survived, making them the only living family that could trace their bloodline back to that of Old Valyria and the Valyrian Freehold. 12 years prior to The Doom, Aenar Targaryen had a daughter, otherwise known as Daenys the Dreamer, who had a prophetic dream in which Valryia was destroyed. Moved by his daughter’s dream, Aenar left Valyria and relocated his family to Dragonstone, becoming henceforth known as Aenar the Exile. Along with his family, he brought five dragons, four of which died; the remaining dragon, Balerion, would grow to be the fiercest dragon ever known and would be rode many years later by Aegon the Conqueror in his Conquest of the Seven Kingdoms.
12 years after relocating to Dragonstone, the visions of Aenar’s daughter, Daenys, came true and Valyria was wiped out in one day, making Aenar and his family the last remaining Targaryens and their dragons the last remaining dragons. Had Aenar not acted on his daughter’s visions, the Targaryen bloodline, as well as their dragons, would have been wiped out forever.
For the next 100 years, the Targaryens lived uneventfully from Dragonstone as they bred from incest and sought to grow their Targaryen family. That is, until Aegon the Conqueror was born and set upon his Conquest of the Seven Kingdoms, alongside his sisters Rhaenys and Visenya.