The Century of Blood, sometimes called the Bleeding Years, is the period of roughly one hundred years that followed the Doom of Valyria and the collapse of the Valyrian Freehold. This period is noted for its chaos, war, bloodshed and political instability in both Essos and Westeros. The collapse of the Volantene hegemony in south-western Essos and the conquest of Westeros by Aegon I Targaryen mark the end of this period.
Over a period of five thousand years (according to myth and tradition), the city-state of Valyria had established a sizable empire extending across much of western Essos, stretching from the Narrow Sea to the lands east of Slaver's Bay. Their dragons allowed them to control this vast territory without threat, defeating every attempt by other powers to challenge them.
In just one night, the Valyrian Freehold was destroyed in a titanic series of volcanic eruptions and earthquakes. The Valyrian Peninsula was shattered, with entire towns and cities destroyed. Vast ashfalls buried many of the other cities, including Valyria itself, whilst most of the Valyrian dragons (who traditionally laired on the very volcanoes which had erupted) were slain in the chaos. The heart of the Valyrian empire was torn out, leaving the colonies to fend for themselves.
In the aftermath of the Doom, Valyria's largest surviving city was Volantis, located several hundred miles to the north-west at the mouth of the Rhoyne. The Volantenes moved to preserve the Freehold, declaring Volantis to be the new capital of the Freehold and demanding that the other colonies and cities accept this. Instead, Pentos, Myr, Lys, Norvos, Qohor, Tyrosh and Lorath all declared themselves independent city-states. The mysterious 'secret city' of Braavos also revealed itself for the first time, helping the other cities gain and maintain their independence.
Furious, the Volantenes launched a military campaign to retake the so-called Free Cities in the name of Valyria. Their armies marched north and west and seized Myr, whilst a fleet sailed south and west to secure Lys. Both cities were ruled as colonies of Volantis for several generations. In this time Volantis attempted to secure an alliance with the Targaryens, the only dragonlords to survive the Doom, but they rebuffed all such overtures.
Despite this, Volantis continued its plans to reforge the Freehold. They launched a fresh campaign against Tyrosh. Tyrosh called for aid and was rewarded when Braavos and Pentos both responded, sending fleets and armies to aid the beleaguered city. Myr and Lys both rose in rebellion, and even King Argilac Durrandon of the Stormlands in Westeros sent aid. Aegon Targaryen, Lord of Dragonstone, also lent his aid to the enemies of Volantis. The result was a humiliating defeat for the Volantenes, who lost their armies and fleets in a series of major defeats. Another fleet, sent into the Smoking Sea in an attempt to reclaim ruined Valyria, was also lost.
The Volantene hegemony was broken and the "tigers" of Volantis, those politicians who favoured military action, were deposed by the "elephants", who favoured using their city's tremendous power and influence to instead dominate trade and mercantile activity along the Summer Sea.
Unfortunately, the Volantene expansion and then retreat led to an absence of control and authority in south-western Essos, most particularly in those lands lying between Myr, Lys and Tyrosh. This region rapidly became disputed between the three cities (and, later, Volantis as well), leading to a state of near-permanent conflict. As a result this region became known as the Disputed Lands, the name it retains to this day.
Valyria's power had kept the primitive tribes-people of central Essos in check for millennia, but with Valyria's fall these tribes began to grew more powerful. The greatest of these, the Dothraki, overwhelmed and conquered all the rest, organising themselves into vast khalasars of tens of thousands. They laid waste to many cities (some of them Valyrian colonies) in their territory, and raided the surrounding regions with ferocity. The kingdom of Sarnor was destroyed and cities such as Meereen were only spared because they agreed to pay tribute to the Dothraki khals.
The Dothraki's aggression was halted at the Battle of Qohor. The newly-independent city of Qohor, aware of the Dothraki's growing power and westward expansion, hired three thousand Unsullied from Astapor to help defend the city. They also hired the Bright Banners and Second Sons - mercenary companies - to augment the defence. Khal Temmo brought fifty thousand Dothraki, half of them warriors, against the city. In a pitched battle the mercenary companies were routed, but the Unsullied held against multiple charges. Twelve thousand Dothraki were killed, including Temmo, before the Dothraki gave up the attack. They saluted the Unsullied as worthy foes by cutting off their braids and throwing them at the warrior-eunuchs' feet. They then withdrew from the field, leaving Qohor untaken.
At least sixteen major cities were destroyed by the Dothraki during the Century of Blood. These were the cities of Essaria, Sarys, Sarnath, Sallosh, Gornath, Kasath and Sathar in Sarnor; Vaes Leisi on the north coast; the Ibbenese colony of Ibbish on the Shivering Sea; the Ghiscari cities of Hazdahn No and Ghazdaq to the south-west (of Vaes Dothrak); and the cities of Vaes Efe, Vaes Mejah, Adakhakileki and Yinishar to the south and south-east. The number of smaller towns and villages destroyed in this period is likely beyond calculation.
Whilst not directly linked to the Doom, the century following it was a noted time of bloodshed in the Seven Kingdoms as well. The ironborn invaded and conquered the Riverlands, establishing a powerful new kingdom which threatened the security of its neighbours, particularly the Stormlands to the south-east. The Dornish were also engaged in conflicts with the Reach as well, to the point that Princess Mariya Martell of Dorne attempted to engage Aegon Targaryen in a military alliance against them, to no avail.
This period of instability in Westeros was ended in the War of Conquest, when, in just two years, Aegon Targaryen and his sisters conquered the continent in a decisive military campaign, failing to secure only Dorne.
The Century of Blood established a new balance of power in the known world following Valyria's collapse. The Free Cities were established and the Seven Kingdoms weakened to the point where the Targaryens could conquer them. The Dothraki emerged and rose to power, whilst new powers such as Braavos began making themselves known.