Burning of the Sept of Remembrance

"Maegor...mounted the black dragon, Balerion, and burned down the sept in King's Landing while the Faith Militant were inside at morning prayers. The screams of the burning and dying men were said to echo throughout the city."

- High Sparrow

The Burning of the Sept of Remembrance was a massacre during the Faith Militant uprising.

History
When the Faith Militant rose in revolt against House Targaryen, they overwhelmed the new capital city King's Landing, causing the weak King Aenys Targaryen to flee to Dragonstone - where he soon died from the stress. Aenys's half-brother Maegor Targaryen then usurped the crown and returned to King's Landing, where he challenged the Faith Militant's champions to a trial of seven to prove his right to rule.

After emerging victorious King Maegor Targaryen, riding the great black dragon Balerion, destroyed the entire