Mad King's Slaughter

The Mad King's Slaughter refers to the mass murder of most established nobles in the Seven Kingdoms, orchestrated by the Mad King Aerys II Targaryen.

The Doomed Summit
Robert's Rebellion was in full swing when the King, seeing he had no chance of winning the conflict, called for the Grand Summit. The great Houses of Westeros, along with the majority of their retainers, all attended what was designed to be the grandest feast and conference ever held.

However the Mad King had stockpiled wildfire under the Great Sept of Baelor, where the peace negotiations took place. As important people packed themselves into the halls, servants of Aerys detonated the wildfire, destroying the Sept and killing everybody inside.

This catastrophe killed a majority of the landed nobility of the Seven Kingdoms, and destroyed most of the major houses. Houses Martell and Tyrell were the only major houses to survive, and that was only through distant relations. The power vacuum created by the death of so much of the landed gentry of Westeros resulted in the Warlord Years, a brutal period of interregnum and civil war.