The Iron Throne is the throne upon which the King of the Andals and the First Men sits, located in the Great Hall of the Red Keep in the city of King's Landing. Besides the King himself (or Lord Regent) only the Hand of the King may sit on the Iron Throne. The term is also used metonymically to refer to the monarchy that rules the Seven Kingdoms and the authority of the King (e.g. "Rebellion against the Iron Throne").
The Iron Throne was forged at the order of Aegon the Conqueror, the first of the Targaryen Kings, who conquered six of the seven independent kingdoms of Westeros and unified them under his rule - the seventh kingdom of Dorne was later joined through a marriage alliance. The throne was allegedly forged from the 1,000 swords that had been surrendered to Aegon in the War of Conquest by the lords who had offered their fealty, though the actual number of the swords is less than two hundred. These were subsequently melted down by the fiery breath of Aegon's dragon, Balerion the Black Dread, then beaten and bent into a throne of imposing appearance.
At the end of Robert's Rebellion, during the Sack of King's Landing, as the Lannister army overran the city outside of the Red Keep, King Aerys II Targaryen - the Mad King - refused to surrender, and secretly ordered the city to be burned to the ground with hidden caches of wildfire. To prevent this, his own Kingsguard Ser Jaime Lannister killed the Mad King in front of the Iron Throne itself. Greatly disturbed at having killed the king he had taken the most sacred oaths to defend, Jaime then sat down on the Iron Throne and gave no thought to the carnage going on outside. Hours later, the main rebel army arrived in the city, and Ned Stark came to the throneroom, where he found Jaime sitting on the throne, and Eddard made Jaime get off of the Iron Throne. Years later, Jaime's sister Cersei chided Eddard that he could have tried to seize the throne then and there, instead of letting Robert take it, but he did nothing. Stark never knew why Jaime really killed King Aerys, and seeing him seated on the Iron Throne like that (apparently, out of arrogance) gave Eddard the incorrect belief that Jaime hoped to seize the throne himself some day.
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