- "When Targaryen fought Targaryen in the civil war called the Dance of the Dragons, an angry mob stormed the Dragonpit, that huge now-ruined vault where Targaryens stabled their beasts. Thousands died, but through sheer numbers and madness, five of the Targaryen dragons lay dead by the morning."
- ―Grand Maester Pycelle
During the Dance of the Dragons, a city-wide riot broke out, and the mob focused their anger on the dragons which were the source and symbol of power for the Targaryen kings. The mob - a vast wave numbering in the thousands - converged on the Dragonpit atop Rhaenys's Hill, the giant domed stable where the dragons were kept when not in use.
Many thousands of rioters died that night, but through their sheer numbers they managed to kill five dragons. The Dragonpit itself was left in ruins when the dragon, Dreamfyre, crashed into the ceiling in an attempt to escape, bringing down the entire structure, killing itself and several hundreds of rioters.
In the books
In the A Song of Ice and Fire novels, the Dragonpit is a huge, cavernous building that sits atop Rhaenys' Hill in King's Landing. It is located on the opposite side of the city from Visenya's Hill, atop which stands the Great Sept of Baelor. Flea Bottom is located on the south side of the hill, and the Street of Silk (the red light district where Chataya's brothel is located) is on the northern side.
During the early years of the city, a great Sept was built on Rhaenys's Hill known as the Sept of Remembrance which used to be the main Sept in King's Landing. During the Faith Militant uprising, about forty years after the Targaryen Conquest, Maegor the Cruel mounted on Balerion the Black Dread incinerated the Great Sept with dragonflame. Maegor later decreed that a large domed structure would be built on the hill, a "stable for dragons".
Not long before Maegor had finished construction on the Red Keep, after which he had all of the builders killed to preserve the secrets of the hidden passages within it. Understandably, few stonemasons and laborers volunteered to build the Dragonpit: instead, prisoners from the city's dungeons were press-ganged into construction labor. They were supervised by architects from Myr and Volantis.
Thirty knights could ride abreast into its entrance. Forty large undervaults were carved deep under the hill, intended for one dragon to nest in each, though there were never that many Targaryen dragons at once. There were only 20 during the Dance, and much fewer during Maegor's time. Apparently he had the foresight to anticipate the number of dragons to grow in the future. Inside of the dome the walls were lined with stone benches which could comfortably seat 80,000 people, so it was used for public spectacles such as the funeral ceremony for King Jaehaerys I Targaryen. When living dragons still nested beneath the dome, light would shine through the windows at night. Subsequent generations of dragons grew smaller. Some said it was because they were contained in the Dragonpit, though others scoff that by the same logic men who live in small houses should grow smaller. Many dragons continued to nest in the sides of the Dragonmont volcano on Dragonstone island. Dragons could be stabled in the Dragonpit, but nests for their eggs needed the heat of the volcano.
During the Targaryen civil war known as the Dance of the Dragons, King Aegon II's coronation took place in the Dragonpit. Later, after King's Landing was captured by Queen Rhaenyra, the Dragonpit was destroyed during the Storming of the Dragonpit. When tens of thousands of crazed and starving smallfolk led by the deranged prophet known as the Shepherd stormed the dragonpit to kill the dragons within. Five Targaryen dragons (Shrykos, Morghul, Tyraxes, Dreamfyre, and Syrax) and thousands of smallfolk were killed. The Dragonpit was reduced to flaming ruins.
So few dragons survived the Dance that the Dragonpit was never rebuilt, but left in ruins for the next 170 years until the War of the Five Kings. The last dragon, a stunted creature, died only 22 years after the Dance, so there was simply no need to maintain it.
During the Great Spring Sickness so many people died so quickly that there was no time to bury the bodies. Instead they were piled up in the Dragonpit and when the corpses were 10 feet deep the Hand of the King, Lord Brynden Rivers, ordered the pyromancers to burn them. The light of the fires shone through the windows and by night citizens could see the dark green glow of wildfire all through King's Landing.
Shortly after the outbreak of the War of the Five Kings, some whores use the Dragonpit as a place to entertain their customers and one of them, along with their patron, falls through the floor. There they find a hidden stash of wildfire, placed by Lord Rossart during Robert's Rebellion as part of the Mad King's Wildfire plot. The pyromancers could never have produced such a vast quantity of wildfire on such short notice, but the hidden caches allowed Tyrion to implement his plan during the Battle of the Blackwater to destroy Stannis' fleet.