- "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
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 the house Strong surrendered their weapons and lowered their banners scarcely 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
The rival claims of Queen Rhaenyra Targaryen and King Aegon II Targaryen led to civil war in the Seven Kingdoms. Rhaenyra's blacks had more dragons than Aegon's greens, but most of their strength was focused around Dragonstone instead of mainland Westeros. With House Tully effectively neutral because of disagreement between Lord Grover Tully and his grandson, Ser Elmo, Rhaenyra's husband, Prince Daemon Targaryen, suggested Harrenhal as a rallying point for potential supporters in the riverlands. Larys Strong, Lord of Harrenhal, was a green supporter of Aegon, however.
While Jacaerys and Lucerys Velaryon left Dragonstone to seek allies and Corlys Velaryon closed Blackwater Bay, Rhaenyra allowed her husband to command the assault on Harrenhal.
Because Lord Larys Clubfoot served as Aegon II's master of whisperers in King's Landing, the elderly Ser Simon Strong served as Larys's castellan at Harrenhal. Prince Daemon flew on his dragon, Caraxes, to the top of Harrenhal's Kingspyre Tower, where Harren the Black had died. Recalling the burning of Harrenhal by Aegon I Targaryen and Balerion the Black Dread, which had extinguished House Hoare, Simon decided to strike his banners and surrender the great ruined castle to Daemon. Simon and his grandsons were among a dozen hostages taken by Daemon, and the blacks requisitioned the wealth of House Strong.
The loss of Harrenhal shocked Aegon II and encouraged the blacks, who soon after won at the Burning Mill and Stone Hedge. House Blackwood and other river lords gathered at Harrenhal, which was where Daemon approved of Blood and Cheese to avenge the death of Rhaenyra's son, Lucerys.
When Daemon learned that Ser Criston Cole and Prince Aemon Targaryen had left King's Landing to retake Harrenhal, the blacks abandoned the castle and the rivermen marched to meet the westermen of Lord Jason Lannister at the Red Fork. While Cole and Aemond retook Harrenhal without bloodshed, their absence contributed to the fall of King's Landing to Daemon and Rhaenyra. The fates of Simon Strong and his grandsons are unknown, although the line of Lord Larys Strong ended during the Hour of the Wolf.