Brotherhood Without Banners

The Brotherhood is an organisation or group working against Lannister interests in the Riverlands at the time of the War of the Five Kings.

Season 2
At Harrenhal, the Lannister torturer known as the Tickler interrogates prisoners and demands to know where the Brotherhood is and what they are planning.

In the books
In the Song of Ice and Fire novels a group of soldiers, knights and armed freemen known as the Brotherhood Without Banners forms to help protect innocents from the depradations of the war (from both Lannister and Stark forces). By a comparable point in the novels, the Brotherhood Without Banners has become an irritant to the Lannister army at Harrenhal and Ser Gregor Clegane's men are trying to find them.