Kingswood Brotherhood

The Kingswood Brotherhood was a band of outlaws, cutthroats, bandits and exiled outcasts who plagued the Kingswood, a forest south-east of King's Landing, approximately twenty years before the series begins. They were wiped out at the order of King Aerys II Targaryen, the Mad King. Their leader, Simon Toyne, was killed by Ser Barristan Selmy in battle, whilst Jaime Lannister, then a squire of fifteen years, killed another member of the band.

In the books
In the Song of Ice and Fire novels, the Kingswood Brotherhood was founded by the brothers Toyne, two outcasts whose brother, a member of the Kingsguard, had been killed by King Aegon IV Targaryen for sleeping with one of his mistresses. They ambushed and killed Ser Aemon Targaryen, the infamous Dragonknight, before disappearing into the vastness of the Kingswood. They and their descendants troubled the realm for several generations before Ser Barristan Selmy led the force of knights and soldiers who finally wiped them out.