- "Rhaenyra's forces struck yet another blow. The moment the Lord of Harrenhal saw Daemon Targaryen circling the castle on his dragon Caraxes, he struck his banners and surrendered. The Blacks now had the strongest castle in the Riverlands."
- ―Viserys Targaryen
The Assault on Harrenhal was a major event during the Dance of the Dragons. It was the first victory of the Blacks, in which Daemon Targaryen took control of the castle of Harrenhal, the largest castle in Westeros and one of the main fortresses of the Greens side.
After the coronation of Aegon II, the claimant to the throne Rhaenyra only had control of the castle of Dragonstone and of the dragons there lodged. When she send her sons to seek support from the Great Lords of Westeros for her cause, her third son Lucerys was killed in an ambush by her brother Aemond, so in revenge her husband Daemon hired two murderers who slaughtered Aegon's son Jaehaerys Targaryen in the Red Keep, ending any possibility of peace between the two Targaryen factions.
While the sons of Rhaenyra continued to seek the support of new vassals, Daemon directed his dragon Caraxes to the Gods Eye to take Harrenhal, the largest castle in all of Westeros. The garrison of house Strong surrendered their weapons and lowered their banners scarcely when they saw the silhouette of the dragon in the sky, and Daemon took control of the stronhold in one night.
After the surrender of Harrenhal, the houses Stark and Arryn opted for Rhaenyra and gave to her their forces. Furious, Aegon deposed his uncle Ser Otto Hightower as King's Hand, whose vision of the war until that moment had been mainly diplomatic and replaced him with the Lord Commander of his Kingsguard Ser Criston Cole. As his last act of government, Ser Otto be asked for the support of Triarchy of Lys, Myr and Tyrosh, old enemies of Daemon, to carry out a naval attack against Dragonstone.
In the books
In the A Song of Ice and Fire novels, the Assault on Harrenhal marked the opening of the hostilities of the Dance of the Dragons. Prince Daemon Targaryen suggested that Harrenhal would be an effective rallying point for potential allies of the Blacks in the Riverlands, and he led the force that conquered the castle for his wife Queen Rhaenyra Targaryen.
Lord Larys Strong, the Lord of Harrenhal, was a supporter of the Greens, but as he was serving as Master of Whisperers to King Aegon II Targaryen in King's Landing, he named the elderly Ser Simon Strong as his castellan in his absence. Daemon flew his dragon Caraxes to the top of Kingspyre Tower, which reminded Simon of the Burning of Harrenhal and caused him to surrender the castle to Daemon. Daemon took a dozen hostages, among them Simon and his grandsons, and obtained the wealth of House Strong.
House Blackwood and other riverlords gathered at Harrenhal, where Daemon chose Blood and Cheese to assassinate on of King Aegon II Targaryen's sons in retaliation for Prince Lucerys Velaryon's death in the Dance Over Storm's End. Aegon sent Ser Criston Cole and Prince Aemond Targaryen to retake Harrenhal and the Blacks abandoned the castle when they learned of their approach. The rivermen traveled to the Red Fork to meet with Lord Jason Lannister and his men. Criston and Aemond took the castle without bloodshed, but their absence from the capital allowed the Blacks to execute the Fall of King's Landing. Aemond had Simon and his grandsons executed, but spared Alys Rivers.