When Gregor Clegane was found to be ravaging the Riverlands, Lord Eddard Stark as Hand of the King, sent Ser Beric with a force of a hundred men to find him and carry out a sentence of death, but it was Beric who was ambushed instead. He died in the Riverlands in 300AL at the hands of Clegane, but was resurrected by the Red priest, Thoros of Myr.
He becomes the leader of the Brotherhood Without Banners. As Eddard and Robert are dead and the Lannisters hold the throne. Ser Beric and his men decide that they have no king and will fight for the common people instead. They become outlaws, but punish injustice and use money they steal to buy food for starving villagers.
However, he permanently gives up his life to resurrect Catelyn Stark, who is found dead in the Trident after the Red Wedding. Now known as Lady Stoneheart, she becomes the new leader of the Brotherhood upon his final death.