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Thoros of Myr is a Red Priest of the Red God R'hllor, from the Free City of Myr. He was the youngest of eight children, so was given to the Red Priests. However though he became one he was never overly pious, enjoying fighting, drinking, and women. He was sent to King's Landing to try converting Aerys II, due to the Mad King's obsession with fire, but failed. Thoros remained when Robert Baratheon took the throne and became a frequent drinking companion of the King.
When Gregor Clegane "the Mountain that Rides" began raiding the Riverlands, Thoros is among the 100 men under Beric Dondarrion that Eddard Stark sends to arrest Gregor. However they are ambushed at the Mummer's Ford and Beric is wounded by the Mountain's lance. Beric's squire Lord Edric Dayne pulls from the water and stands over him while the fighting happens, though Beric dies. Thoros, who is a friend of Beric, performs the last kiss on Beric, a rite of R'hllorism. To his astonishment Beric is brought back to life. He rallies the survivors to fight the Lannisters' invasion of the Riverlands. However when King Robert dies the Lannisters usurp the throne, making Beric and his men outlaws. Eddard Stark is executed by the Lannisters' King Joffrey Baratheon. Eddard's son Robb Stark arrives in the Riverlands to fight the Lannisters, and on hearing of his father's death the Northmen and Rivermen at Riverrun proclaim him King in the North.
The war between the Lannisters and Starks leaves the Riverlands in ruins. Beric raids the Lannisters' foraging parties. Though some of his men die, others join Beric's force, who become the Brotherhood without Banners. Though Beric and his men were originally sent to fight the Lannisters, the Brotherhood fights all who oppress the smallfolk. Stories develop about Beric, claiming he can't be killed, as several men claim to have killed him but he keeps returning.
Beric is killed five times, and each time Thoros revives him. Beric meets Arya Stark, the younger daughter of Ned Stark, and Gendry, a bastard of Robert Baratheon. They capture Gregor Clegane's brother Sandor Clegane, "the Hound" and Beric has a trial by combat with him, in which the Hound kills him. For the sixth time Thoros brings Beric back.
At a Sept, the Brotherhood, including Beric and Thoros, with Arya, encounter a band of Brave Companions, a vile sellsword company who originally fought for Tywin but have turned their cloaks to the Starks. This band is being led by Septon Utt, a pedophile rapist and murderer. The Brotherhood hang him for his crimes. One of the novices objects to Thoros praying to R'hllor under their roof, which Thoros agrees to. Thoros tells Beric reviving him becomes harder each time. Beric says he can barely remember his old life, he says he couldn't find his castle or remember the color of the hair from the woman he was pledged to marry.
In what becomes known as the Red Wedding, Robb Stark, his mother Catelyn Stark, and most of their army are massacred and Catelyn's brother Edmure Tully captured by their treacherous vassals, the Freys and Boltons, who have secretly joined the Lannisters. Catelyn is stripped and thrown in the river after a day and a night. Arya, while warging into her direwolf Nymeria, finds her mother and drags her out of the water. She is found by the Brotherhood and Harwin, the former Master of Horse at Winterfell, begs Thoros to bring her back. However Thoros claims she has been dead too long. Beric then performs the last kiss on her, and Catelyn rises, while Beric dies a seventh and final time. Leadership of the Brotherhood is taken by Catelyn, who becomes known as Lady Stoneheart. Stoneheart's mission, to take vengeance on all those she believes connected to the Lannisters, leads the outlaw band to fracture. Thoros remains with Stoneheart, however he becomes more cynical, and starts taking up drinking again.
Brienne of Tarth, her squire Podrick Payne, and Ser Hyle Hunt arrives in the Riverlands, searching for Sansa Stark and Arya Stark. However at the Crossroads Inn they encounter a band of the Brave Companions. Brienne fights them to protect the war orphans at the Inn but is wounded, however she is saved by Gendry. Brienne is taken to the Hollow Hill, the Brotherhoods' base, where she meets Thoros. He tells her the Brotherhood have lost their original sense of justice, before Brienne is taken to Stoneheart.