Doom of Valyria
The Doom of Valyria, often called the Doom, was a cataclysm of an unspecified nature that caused the collapse of the Valyrian Freehold, which had been prospering for five thousand years. It occurred approximately a century before Aegon's Conquest of Westeros, in 114 BC. Spells, knowledge, and recorded history were lost in the Doom, and the Century of Blood followed the disaster.
Twelve years before the Doom, Lord Aenar Targaryen's maiden daughter, Daenys the Dreamer, had a powerful prophetic dream along with visions that Valyria would be destroyed. Foreseeing the Doom, she convinced her father to leave Valyria before the cataclysm. Aenar set sail, moving House Targaryen and their five dragons to the island Dragonstone. Other Houses of the narrow sea of Valyrian descent include the Velaryons of Driftmark and and the Celtigars of Claw Isle.
On the day of the Doom, every hill for five hundred miles exploded, filling the air with ash and smoke and fire, which killed even dragons. Earthquakes destroyed palaces, temples, and towns, while lakes boiled or turned to acid. The Fourteen Flames, the fiery mountains of Valyria, sent molten rock a thousand feet into the air, and red clouds rained down dragonglass. The cataclysm fragmented the Valyrian peninsula surrounding Valyria into numerous smaller islands and creating the Smoking Sea between them. East of Valyria, Velos and Ghozai on the Isle of Cedars were destroyed by a tsunami.
It is commonly believed that the Doom was a natural calamity caused by the eruption of the Fourteen Flames, although some septons believe the polytheistic Valyrians delved too deep to the seven hells. Alternatively, Septon Barth and some maesters believe that Valyrian spells controlling the Fourteen Flames faltered. Other explanations include the curse of Garin the Great, the fire of R'hllor, or infighting dragonlords assassinating fire mages.
Much of Essos fell into chaos after the Doom. During the Century of Blood, the Volantenes unsuccessfully considered themselves the heirs to Valyria, often waging war against other Free Cities. The Dothraki conquered many of the lands surrounding the Dothraki sea, and the last Ibbenese king was thrown down. The Slaver Cities of Slaver's Bay regained their independence.
Some dragonlords in Lys and Tyrosh survived the Doom, but they were soon killed along with their dragons. The dragonlord Aurion declared himself Emperor of Valyria and led a host from Qohor toward Valyria, but he and his followers were not seen again. Tommen II Lannister, King of the Rock, vanished while sailing to Valyria with his blade, Brightroar, and Gerion Lannister later disappeared while searching for Brightroar. It is thus said that no one has returned from a journey to the ruins of Valyria, and even the most hardened sailors shudder at the thought of sailing to ruined Valyria.
Many of Valyria's secrets, such as magic, the method for creating Valyrian steel, and ancient records were lost in the Doom. The disaster is represented in art across the known world. Songs are sung of the Doom.
In 54 AC during the reign of Jaehaerys I Targaryen, the Princess Aerea Targaryen disappeared from Dragonstone with the dragon Balerion, no one knew where they went, many puzzled by the lack of sightings. Balerion returned to King's Landing in 56 AC, descending into the courtyard of the Red Keep with the sickly and afflicted Princess Aerea barely clinging to him. At first no one recognised her, she was cadaverous in appearance, what clothes left on her where nothing more than charred rags. Her hair a tangled mess, her eyes bloody, she trembled as she tried to take a step and could barely speak and only got a few words out before collapsing. The Princess was carried to Maester Benifer,Septon Barth aided the maester in trying to help the girl, but they're efforts failed.
Septon Barth believes that when Aerea became Balerion's rider, she lacked the iron will of the dragons previous riders (Aegon and Maegor) and could not control Balerion. Instead of asking where Aerea had taken Balerion, the question where Balerion took Aerea should have been asked. Barth believes Balerion returned to where he was born, Valyria. Balerion was the only Dragon living to have been born in Valyria before the Doom came , and was believed to have gone back to his birth place. Aerea was powerless to control him. Princess Aerea was infected by an insidious parasitic wrym while there. When she returned, it was if she was burning from within, she died in agony. Barth who witnessed her death was haunted by what the girl went through. and disturbed by some of the things she said while dying. 
Barth though was even more horrified when he noted Balerion bore scars he previously did not have, the dragon also had a huge jagged rent down his left side almost nine feet long, fresh blood still dripped from the wound, hot and smoking. Barth ponders what could have done that to a dragon as large and powerful as Balerion. After learning all this information from Barth, King Jaehaerys had Balerion secured in the Dragonpit and placed under guard. Jaehaerys also learning of his nieces fate and the "horrors" that infected her, declared that attempting to sail to Valyria was a crime punishable by death, and any ship that travelled too close to Valyrian Islands or that sailed the Smoking Sea was forbidden to dock in any port in Westeros. 
A Game of Thrones
A Storm of Swords
A Feast for Crows
Euron Greyjoy claims to have reaved all over the world during his exile, including a trip to the feared ruins of Valyria. During the kingsmoot he claims to have found the dragon horn among the smoking ruins that were Valyria, where no man has dared to walk but him.
A Dance with Dragons
While in Meereen the merchant prince Xaro Xhoan Daxos gifts Daenerys a tapestry that has been in his family vaults since before the Doom that took Valyria. The old tapestry depicts the Valyrian peninsula before its destruction.
Valyria is accursed, all men agree, and even the boldest sailor steers well clear of it's smoking bones...but we would be mistaken to believe that nothing lives there now. The things we found in Aerea Targaryen live there now, I would submit...along with such other horrors as we cannot begin to imagine.
Every man there knew that the Doom still ruled Valyria. The very sea there boiled and smoked, and the land was overrun with demons. It was said that any sailor who so much as glimpsed the fiery mountains of Valyria rising above the waves would soon die a dreadful death, yet the Crow's Eye had been there, and returned.
- thoughts of Victarion Greyjoy
- thoughts of Tyrion Lannister
Behind the Scenes
- A Storm of Swords, Chapter 23, Daenerys II.
- A Feast for Crows, Chapter 11, The Kraken's Daughter.
- The World of Ice & Fire, Ancient History: The Doom of Valyria.
- The World of Ice & Fire, The Reign of the Dragons: The Conquest.
- A Dance with Dragons, Chapter 33, Tyrion VIII.
- A Dance with Dragons, Chapter 56, The Iron Suitor.
- A Dance with Dragons, Chapter 14, Tyrion IV.
- The World of Ice & Fire, Beyond the Free Cities: The Grasslands.
- The World of Ice & Fire, Beyond the Free Cities: Ib.
- The World of Ice & Fire, Ancient History: The Rise of Valyria.
- A Storm of Swords, Chapter 32, Tyrion IV.
- A Feast for Crows, Chapter 29, The Reaver.
- A Game of Thrones, Chapter 23, Daenerys III.
- The World of Ice & Fire, Ancient History: Valyria's Children.
- A Game of Thrones, Chapter 3, Daenerys I.
- A Storm of Swords, Chapter 60, Tyrion VIII.
- Fire & Blood, Birth, Death, and Betrayal Under Jaehaerys I.
- Fire & Blood, Jaehaerys & Alysanne- Their Triumphs & Tragedies.
- A Feast for Crows, Chapter 19, The Drowned Man.
- A Dance with Dragons, Chapter 16, Daenerys III.
- A Storm of Swords, Chapter 66, Tyrion IX.
- So Spake Martin: Dromen & Demonen Chat (April 23, 2002)
- George R.R. Martin Talks Season Two, 'The Winds of Winter,' and Real-World Influences for 'A Song of Ice and Fire' (March 26, 2012)