Eddard Stark sent Lord Beric Dondarrion and a company of household knights and men-at-arms into the riverlands to bring the outlaw Gregor Clegane to the king's justice; the war that followed swallowed the lord who gave the order before the men he sent could return. Left leaderless and cut off in country crawling with soldiers of every banner, Beric's company turned to striking at whoever preyed on the smallfolk — Lannister, Frey, or freebooter alike — and took to calling themselves the brotherhood without banners, having sworn off every house's colors but their own cause.
Under the peculiar ministry of the red priest Thoros of Myr, Beric Dondarrion is dragged back from mortal wounds again and again, each return costing the man a little more of himself than the last; the brotherhood in this period functions as judge, jury, and gallows for lords and smallfolk alike, trying prisoners "before the Lord of Light" rather than any crown's law, a practice that has earned it both blessing and curse from those it has judged.
Beric's final resurrection is spent not on himself but on a drowned and murdered noblewoman pulled from the Green Fork, and command of the surviving brotherhood passes to a captain this chronicle declines to name in full. What that band does under her silent, rope-scarred leadership belongs to the darker and more recent chapters of the published record, and is left there rather than repeated here.