The Brotherhood Without Banners is an outlaw organization also known as the Knights of the Hollow Hill. Their goals include fighting the Lannisters and protecting the common people of the the Riverlands from depredation by warriors fighting the War of the Five Kings.

History
The Brotherhood was originally founded by Beric Dondarrion and Thoros of Myr. In response to the sacking of several Riverland villages, Hand of the King Eddard Stark charged them to bring the Lannister bannerman Gregor Clegane to justice in the name of King Robert. Beric, having the command of the party, led his men into an ambush and was killed. Afterwards, Thoros unintentionally resurrected Beric during a common burial ritual of his god R'hllor. The revived Beric swore to uphold his oath to King and resumed his quest for Gregor's head. Once Robert died and the Lannisters assumed control of the Iron Throne, Beric's men became an outlaw organization.

As outlaws, Beric's men formed the Brotherhood as a way to uphold the spirit of Robert's decree and protect the people. With the help of local communities, Beric began leading successful ambushes on Gregor Clegane's men and other soldiers of all sides of the ongoing conflicts. Due to the odds against him, however, Beric died several more times, but was resurrected each time when Thoros performed his burial ritual. Beric developed a heroic reputation and was dubbed "the Lightning Lord" for his Dondarrion arms and his lethal ambushes. His various deaths and inexplicable reappearances fed the Brotherhood's mystique. Despite numerous opportunities, however, the Brotherhood could not kill Gregor.

Groups of the Brotherhood managed to capture Arya Stark and Sandor Clegane and bring them to Beric's hideout at the same time. Beric decided to hold Arya until they could ransom her to her Tully relatives. He put Sandor on trial for Lannister atrocities. On Arya's eyewitness testimony, Beric convicted Sandor of murdering a butcher boy, but allowed him to choose a trial by combat. He fought Sandor personally with a flaming sword, but was killed. The Brotherhood allowed Sandor to leave, though they seized his possessions in exchange for a deed. Thoros resurrected Beric once again, and later Arya was captured by Sandor.

After the Red Wedding, the Brotherhood discovered Catelyn Stark's corpse, upon which Beric performed R'hllor's burial ritual. He exchanged his life for hers, dying in truth and resurrecting Catelyn's half-decayed body. Though rendered nearly mute from an unhealed throat injury, Catelyn took the name Lady Stoneheart and became the Brotherhood's new de facto leader.