Destruction of the Great Sept of Baelor

To make sure King's Landing could burn if it ever fell into enemy hands, Aerys II had the Guild of Alchemists plant wildfire across the city before Robert's Rebellion ended. He ordered his Hand, Lord Rossart, to light it, but Jaime Lannister killed Rossart and then the king himself, and the city was spared.

In the years that followed, the alchemists dismantled most of those caches. One, hidden beneath the Great Sept of Baelor, went unnoticed and stayed there, forgotten, waiting.

When the War of the Five Kings left the realm in ruins, a religious movement called the Sparrows gained ground among the capital's poor, led by a man who called himself the High Sparrow. With Tywin Lannister dead and no father or brothers left to rein her in, Cersei took control of the small council in her son Tommen's name.

Fearful of the Tyrells' rise and of what Margaery meant to her son, Cersei found an unlikely ally: she named the Sparrow High Septon and gave the Faith Militant back the power it had lost generations before. With that force behind her, she had Loras Tyrell accused of sodomy; Margaery tried to defend him at his hearing, but one of Loras's lovers confirmed the charges, and the Sparrows locked up both siblings.

The move backfired. Her cousin Lancel confessed to the High Sparrow that he had slept with her and helped kill King Robert, and Cersei ended up a prisoner alongside the people she had tried to bring down, charged with adultery, regicide, and incest. She admitted only to the adultery, and to return to Tommen she had to walk naked from the Sept to the Red Keep while the city jeered.

With formal trials set for Cersei, Loras, and Margaery, and with Ser Gregor Clegane — revived by Qyburn — as her possible champion, Cersei still had a way out if the trial came down to combat. When Jaime returned to the city, he convinced her, along with their uncle Kevan Lannister and Lady Olenna Tyrell, to attack the Great Sept and stop Margaery from walking the same path of atonement she had.

The assault failed: the High Sparrow had already struck a deal with King Tommen, and Margaery was freed from the penance. Tommen, steered by the Septon, named Jaime Lord Commander of the Kingsguard and sent him away to the siege of Riverrun, and did away with trial by combat as a legal option. With that door closed, Cersei was left without an army of her own and with no ally but Qyburn; when she asked Olenna for help, the old woman answered only with contempt before leaving the city.

On the day set for the trials, Cersei never showed up at the Sept. She had found the forgotten wildfire beneath the tunnels and had candles set beside the barrels, timed to burn down and light the fuse. Her cousin Lancel uncovered the plot after one of Qyburn's "little birds" led him to the tunnels and wounded him badly; the injury kept him from reaching the candles in time, and he was the first to die when the fire reached the barrels.

The blast rose up from the foundations. Margaery realized something was wrong when neither Cersei nor Tommen appeared, and tried to get her family out, but the High Sparrow insisted the trial continue, and the Sparrows wouldn't let anyone leave. Fire tore through the floor of the Sept, the statues of the Seven shattered, and the flames left no one inside alive.

The shockwave leveled everything within a hundred yards, incinerated the crowd waiting outside, and tore a bell loose from its tower, sending it crashing into the street below. From the Red Keep, Cersei watched the fire with satisfaction; beside her, Tommen — kept there on Ser Gregor's orders — watched the Sept come apart without yet understanding what was happening.

Hundreds died that night inside and around the Sept, among them the High Sparrow and most of his followers, ending the power the Faith Militant had clawed back over the capital. Kevan Lannister also died, and with him his son Lancel; of Tywin's children only Cersei, Jaime, and Tyrion were left, and House Lannister stood dangerously thin.

Tommen, undone by his wife's death, took his own life that same night. With no heir to follow him and no one left who could challenge her for the throne, Cersei crowned herself queen of the Seven Kingdoms.

Of all the great houses, the Tyrells had come through the war more intact than most; they lost that all at once, with the deaths of Lord Mace, Ser Loras, and Queen Margaery. Control of the Reach fell to Lady Olenna, the only one left in the family with the standing to hold it.

Olenna answered by withdrawing her support from the Iron Throne and allying with Daenerys Targaryen. The alliance didn't save her: when Lannister forces stormed Highgarden, with House Tarly already on the other side, House Tyrell was wiped out entirely.