Destruction of the Great Sept of Baelor

The Destruction of the Great Sept of Baelor was an event that occurred in King's Landing late in the War of the Five Kings. In order to evade her trial, Queen Cersei Lannister has the Great Sept of Baelor destroyed by wildfire immediately after the trial of Ser Loras Tyrell. As a result, the entirety of House Tyrell is killed, with the exception of Olenna Tyrell who fled the city days before the explosion, as well as all of the Sparrows, including the High Sparrow, and Ser Kevan Lannister, the Hand of the King. Shortly after witnessing the massacre and hearing of these deaths, King Tommen I jumps out of a window to his death. Cersei is later crowned in his place.

A Mad King
During Robert's Rebellion, in which several noble houses rose up in rebellion against King Aerys II Targaryen, the Mad King, Aerys II commissioned the Alchemists' Guild to place caches of wildfire underneath King's Landing. In the event that the rebels were to take the city, he intended to burn it down, killing all of its inhabitants in the process. House Lannister eventually did sack the city in the name of Robert Baratheon. Aerys II then gave the order to his Hand, Wisdom Rossart. However, before the order could be carried out, Ser Jaime Lannister of the Kingsguard murdered Rossart, the Hand of the King, and then Aerys II, saving the city and its inhabitants from total annihilation.

Rise of the Sparrows
The War of the Five Kings saw destruction and death all across the country. In response to this, a cult of the Faith of the Seven formed, the Sparrows, led by a man known as the High Sparrow. Following the death of Lord Tywin Lannister, the Sparrows found their way into King's Landing, preaching their extreme beliefs.