During the Dance of the Dragons, Prince Aemond Targaryen was sent to Harrenhal in order to retrieve his uncle Daemon Targaryen. He took the Green Army and its leader Ser Criston Cole with him. Aemond himself was atop his ancient dragon Vhagar. When they arrived at the fortress, Daemon had already left, causing a celebration among the ranks.
However, Daemon had left in order to lay siege to the capital city of King's Landing. He had flown his dragon Caraxes south in order to meet up with his wife. Unknown to Aemond, Daemon's exit happened as he entered, flying over the waters of the Gods Eye. The population of King's Landing then saw the terrifying shadows of Caraxes and Syrax, about the attack the city and take it. Rhaenyra had come for the Iron Throne. The city was defenseless since that Aemond had taken Aegon II Targaryen's army away from the city, aswell as Vhagar. King's Landing couldn't resist the attack, so the Small Council surrendered Aegon's wife Helaena Targaryen and his mother Alicent Hightower. The King himself had fled the city, despite his wounds and madness after the Siege of Rook's Rest. This allowed Rhaenyra to take her place on the throne. However, after leaving it for the first time, blood trickled down her. There was a rumor that the Iron Throne has spurned her, as it saw Aegon as the rightful King of the Andals, the Rhoynar, and the First Men. However, it was more likely that due to wanted the throne for saw long, she gripped it too tightly, leading to her cuts on the swords that composed the throne.
After discovering that Rhaenyra had taken his family hostage, Aemond burn every town and village that had sweared fealty to her, on his way back to King's Landing. Criston led the army back to the city in order to take it for himself. This also led to his murder by the lords in The Riverlands who had also become Blacks loyalists.
In the books
Prince Aemond Targaryen and Ser Criston Cole gathered their green forces to march on Harrenhal, which left King's Landinglargely defenseless. This created an opportunity that the blacks seized.
Prince Daemon Targaryen on Caraxes and Queen Rhaenyra Targaryen on Syrax flew over the capital and circled their dragons above Aegon's High Hill. The fleet of the Sea Snake, Lord Corlys Velaryon, then arrived to the east of Blackwater Bay.
The sight of Daemon on Caraxes and Rhaenyra on Syrax over King's Landing sent the city into a panic. The Queen Dowager,Alicent Hightower, tried to send riders to bring back Aemond and ravens to leal lords, but her efforts were quickly stopped. Unknown to Alicent was that the gold cloaks, the City Watch of King's Landing, had turned. The gold cloaks were loyal to Daemon who had once been their leader. When he went to dispatch ravens, Grand Maester Orwyle was beaten and bound by gold cloaks as he arrived in his chambers. The seven commanders of the city gates who had been chosen for their loyalty to King Aegon II Targaryen were either imprisoned or killed the moment Caraxes was spotted over the Red Keep. Ser Gwayne Hightower, the second in command of the City Watch, was captured when he tried to sound the alarm. He was brought before Ser Luthor Largent, the commander of the gold cloaks. Gwayne denounced him as a turncloak to which Luthor replied:
Luthor then murdered Gwayne and had the city gates opened to the army pouring in from the Sea Snake's ships. ThirteenHouse Hightower knights and a hundred men-at-arms managed to hold the River Gate for eight hours from both attacks without and within the city. The other six gates, however, fell swiftly and the blacks' army entered the city unmolested through those gates. The city fell in less then a day. Seeing hope was lost, Queen Alicent ordered her forces to surrender.
King Aegon II Targaryen escaped the fall of King's Landing. He was smuggled out by the master of whisperers, Lord Larys Strong, via secret passages. Aegon was protected by two of his Kingsguard knights, Ser Willis Fell and Ser Rickard Thorne, and was accompanied by his children, Princess Jaehaera and Prince Maelor.
Lord Otto Hightower and Lord Jasper Wylde were executed for treason. Queen Alicent Hightower and Princess Helaena Targaryen were held as captives. Ser Tyland Lannisterwas tortured for information on where he had hidden the crown's treasure. Grand Maester Orwyle was confined in the black cells.
Queen Rhaenyra Targaryen seized the Iron Throne, though many lords noted she cut herself upon the throne, starting rumors that the throne had rejected her. At first the population of the city had welcomed Rhaenyra's ascent to the Iron Throne as Aegon II and Prince Aemond Targaryen had not been much loved. In time, however, with more executions happening daily and more taxes being raised to fund the wars, the population began to loathe Rhaenyra, calling her "King Maegor with teats".