The Brotherhood Without Banners is a group of outlaws. The founding members of the group were the surviving knights and soldiers of the men originally sent by Lord Eddard Stark, Hand of the King, to bring Gregor Clegane to justice and demand that Tywin Lannister, head of House Lannister, return to King's Landing to answer for the crimes of his bannerman.
They were defeated and cut off when Robert and Eddard died. These men were led by a revived Beric Dondarrion and Thoros of Myr, and found themselves with no king to pledge alligence to. They decided to act as their own vigilante group, fighting for justice and the common people by robbing from the rich to buy food for the starving.
Lannister forces were torturing villagers for information on their whereabouts.