Wildfire Plot

"The Mad King was obsessed with it. He loved to watch people burn, the way their skin blackened and blistered and melted off their bones. He burned lords he didn't like. He burned Hands who disobeyed him. He burned anyone who was against him. Before long, half the country was against him. Aerys saw traitors everywhere. So he had his pyromancers place caches of wildfire all over the city. Beneath the Sept of Baelor and the slums of Flea Bottom. Under houses, stables, taverns. Even beneath the Red Keep itself."

- Jaime Lannister

The wildfire plot was a plan developed by King Aerys II Targaryen during the latter stages of Robert's Rebellion.

History
As he neared the end of his life, and his mental state deteriorated, Aerys developed an obsession with wildfire. He enjoyed watching people burn, and in his paranoia would burn anyone he saw as an enemy. Eventually, he had Wisdom Rossart, head of the Alchemists' Guild and whom he later named Hand of the King, place caches of wildfire all over King's Landing.

After the death of Prince Rhaegar Targaryen in the Battle of the Trident, Lord Tywin Lannister marched to the capital with an army, promising to protect the city from the rebels led by Lord Robert Baratheon. Both the Master of Whisperers Varys and Ser Jaime Lannister of the Kingsguard urged Aerys bar the gates and refuse Tywin entrance, but Grand Maester Pycelle urged him to let Tywin enter the city. Aerys did, and Tywin's army sacked the city.

Jaime once again urged Aerys to surrender, but the Mad King told him to bring him Tywin's head. He then ordered Rossart to burn the city to the ground. He believed that once the city was destroyed, he would rise from the ashes as a dragon. Before they could go through with this, Jaime killed Rossart and when Aerys tried to flee, Jaime stabbed him in the back and slit his throat to make sure he was dead. At this point, Lord Eddard Stark entered the throne room and discovered Jaime standing over the king he had just killed.

Hallyne, an acolyte at the time, had heard whispers of this plot. Though he did not know the details, he heard that Aerys had engaged the Guild's greatest Wisdoms for an ultimate weapon to use against Robert. He considered it unfortunate that King's Landing fell before it could be used and that many of the Wisdoms disappeared during the sack, whom he believed to be victims of ignorance and envy.

In the books
in the A Song of Ice and Fire novels, the wildfire plot occurs in a similar fashion, but is explored in greater detail.

After the rebel victory of Robert Baratheon at the Battle of the Bells, Aerys was fearful that Robert might actually win the war and enlisted the aid of several members of the Alchemists' Guild in a plot to destroy the city of King's Landing should the rebel forces prove victorious. Jaime was present when this was planned, while the rest of the Kingsguard was absent and went with Crown Prince Rhaegar Targaryen.

Thousands of jars of wildfire were gathered and placed in key positions under King's Landing. Hundreds were placed under the Dragonpit and the Great Sept of Baelor, under every one of the city's gates and even under the Red Keep itself. The pyromancers did their work in secret and did not trust their apprentices with the knowledge.

The rest of the royal family were unaware of this; apparently Queen Rhaella Targaryen's eyes had long been shut to her husband's insanities, and Rhaegar was busy marshalling an army. The Hand of the King, Lord Qarlton Chelsted, grew suspicious of the constant comings and goings of the pyromancers from the Red Keep. When Chelsted discovered the plot, he confronted Aerys. When reasoning, threatening, jesting and even begging on his part failed to dissuade Aerys from this course of action, Chelsted resigned his handship, flinging his chain of office at Aerys's feet. Aerys had him burned alive for this and made the pyromancer Rossart his new Hand of the King.

During the Sack of King's Landing, Aerys commanded Rossart to ignite the wildfire to destroy the city and Jaime to bring the head of his father, Lord Tywin Lannister. Jaime, however, intercepted and slew Rossart to prevent his message from reaching other pyromancers. Jaime then slew Aerys, slitting his throat. A few days later Jaime hunted and killed Garigus and Belis, the other pyromancers involved in the plot.