Doom of Valyria
From A Wiki of Ice and Fire
The Doom of Valyria is the name for the cataclysm of an unspecified nature that caused the collapse of the Valyrian Freehold. It occurred approximately a century before the Targaryen Conquest of Westeros. It is possible that GRRM's inspiration for the Doom was an interpretation of the fall of Atlantis. A century of blood and chaos followed the Doom of Valyria.
Foreshadowing of the Doom
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. Heeding the warning Aenar set sail, moving his entire family and their five dragons to the island Dragonstone.  Closely allied families, including the Velaryons and Celtigars, went with them, settling on other islands in the narrow sea.
The cataclysm hit the Valyrian capital city Valyria, fragmenting the land surrounding the city into numerous smaller islands, creating the Smoking Sea between them. the Freehold was annihilated in a single night of fire and storm, with great earthquakes and volcanic eruptions laying waste to the empire. The area is now described as "demon-haunted." Most people are afraid to go there, saying,
|“||The Doom still rules in Valyria. ||”|
It is said that few survive a journey to the ruins of Valyria. Even the most hardened sailors shudder at the thought of sailing to post-Doom Valyria. The very sea there is said to smoke and boil, and the land is said to be overrun with demons. It is said that any sailor who so much as glimpses the fiery mountains of Valyria rising above the waves will soon die a dreadful death. 
This disaster was dramatic enough that it is represented in art across the world. The entry hall of Khal Drogo’s nine-towered mansion in Pentos has a mosaic of colored glass depicting the Doom of Valyria. His prospective bride Daenerys Targaryen notices it when she enters. Songs are sung of the Doom; a haunting ballad of two lovers dying amidst the Doom of Valyria is sung in High Valyrian at King Joffrey Baratheon’s wedding.
The Day of Doom
Tyrion recalls that it was written that on the day of Doom every hill for 500 miles had split asunder to fill the air with ash and smoke and fire, blazes so hot and hungry that even dragons in the sky were engulfed and consumed. Great rents had opened in the earth, swallowing palaces, temples, entire towns. Lakes boiled or turned to acid, mountains burst, fiery fountains spewed molten rock a thousand feet into the air, red clouds rained down dragonglass and the black blood of demons, and to the north the ground splintered and collapsed and fell in on itself and an angry sea came rushing in. 
|“||The proudest city in all the world was gone in an instant, its fabled empire vanished in a day, the Lands of the Long Summer scorched and drowned and blighted. ||”|
Tyrion thinks to himself,
|“||An empire built on blood and fire. The Valyrians reaped the seed they had sown. ||”|
While aboard the Iron Victory, near the Isle of Cedars, Victarion Greyjoy recalls what he knows of the day the Doom came to Valyria. It is said a wall of water 300 feet high had descended on Velos drowning hundreds of thousands of man, women, and children, leaving none to tell the tale but some fisherfolk who had been at sea and a handful of Velosi spearmen posted in a stout stone tower on the island’s highest hill, who had seen the hills and valleys beneath them turn into a raging sea. 
Fair Velos with its places of cedar and pink marble had vanished in a heartbeat. On the north end of the island, the ancient brick walls and steeped pyramids of the slaver port of Ghozai had suffered the same fate. 
References in the books
A Game of Thrones
A Storm of Swords
A haunting ballad of two lovers dying amidst the Doom of Valyria is sung in High Valyrian at King Joffrey Baratheon’s wedding by Collio Quaynis, although according to Tyrion it is more properly a song for two singers, male and female.
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 amongst the smoking ruins that were Valyria, where no man has dared to walk but him. 
A Dance with Dragons
The tapestry is old, dusty, faded... and huge. It is beautiful and covers half the floor. The seas are a blue silk sea; the lands are green, the mountains black and brown. Cites are shown as stars in gold or silver thread. There is no Smoking Sea; Valyria is not yet an island. Astapor, Yunkai and Meereen are three silver stars beside the blue of Slaver’s Bay. Westeros is distant, toward the far end of the map.
One night the Selaesori Qhoran is sailing closer to Valyria than the crew would like. Tyrion goes above decks for a breath of night air. Above only the brightest stars are visible, all to the west. A dull red glow lights the sky to the northeast, the color of a blood bruise. Tyrion has never seen a bigger moon. Monstrous and swollen, it looks as if it has swallowed the sun and woken with a fever. According to Moqorro the sky is always red above Valyria. Tyrion speculates that it is perhaps the fires of the Fourteen Flames reflected in the clouds. 
Ralf’s Iron Fleet squadron of ships are hit by three big storms, and foul winds between. Red winds out of Valyria that smelled of ash and brimstone, and black winds drove them toward that blighted shore.
References and Notes
- ↑ The full story of the Targaryen Conquest in A SONG OF ICE AND FIRE
- ↑ Chicon 7 Reading (September 02, 2012) So Spake Martin
- ↑ 3.0 3.1 A Feast for Crows, Chapter 29, The Reaver.
- ↑ 4.0 4.1 4.2 4.3 A Dance with Dragons, Chapter 33, Tyrion.
- ↑ 5.0 5.1 A Dance with Dragons, Chapter 56, The Iron Suitor (Victarion 0).
- ↑ A Game of Thrones, Chapter 3, Daenerys.
- ↑ A Feast for Crows, Chapter 19, The Drowned Man.
- ↑ A Dance with Dragons, Chapter 16, Daenerys.