Sack of King's Landing
The Sack of King's Landing was the brutal fall of the Targaryen capital at the close of Robert's Rebellion in 283 AC, when the forces of House Lannister entered the city under false pretenses of loyalty and put it to the sword. The sack ended the rule of the Targaryen dynasty and is infamous for the murders carried out within the Red Keep.
Background
After Prince Rhaegar Targaryen fell at the Battle of the Trident, the Targaryen cause was all but lost. Tywin Lannister, who had held Casterly Rock aloof from the war, marched on King's Landing and presented himself as a friend to the crown. The Mad King Aerys II Targaryen opened the gates, and the Lannister host poured in and sacked the city.
The Murders
Lannister men murdered the royal family. Ser Gregor Clegane killed Princess Elia Martell and her infant son Aegon, and her daughter Rhaenys was slain as well. Aerys II himself was killed in the throne room by his own Kingsguard, Jaime Lannister, who slew the king to stop him from burning the city with wildfire, earning the name Kingslayer.
Aftermath
Robert Baratheon took the Iron Throne, and Tywin Lannister's brutal pragmatism secured the new king's hold on the capital, sealed by Robert's marriage to Cersei Lannister. The murder of Elia and her children, however, left a blood debt that would shape the politics of Dorne and the realm for years to come.