Thoros is a red priest of the Lord of Light and a noted warrior.
Thoros was born on the Free City of Myr. Being an eighth son, Thoros' father had no means to support him and gave him up to the local Temple of the Lord of Light. There, he was raised to become a Red Priest, though he grew up a glutton and lustful man, constantly raiding the kitchens and taking girls to his chambers.
He was tasked by the High Priest to turn future King Robert Baratheon away from the Seven and convert him to the Lord of Light. He was already present in the Red Keep by the time the bodies of Rhaenys and Aegon Targaryen were presented to Robert. Thoros became part of the court and a drinking and whoring companion of King Robert. He also became famous for using a sword coated with wildfire and set ablaze before combat as a way of intimidating his enemies.
He fought for King Robert Baratheon in the Greyjoy Rebellion. He was the first man, followed closely by Ser Jorah Mormont, through the breach during the Siege of Pyke, stabbing and terrifying the ironborn with his flaming sword.
Jory Cassel and Jaime Lannister reminisce about the Siege of Pyke. Jory swears that he will remember Thoros charging through the breach with his blazing sword until his dying day
Thoros of Myr, along with Anguy and other members of the Brotherhood Without Banners, runs into Arya, Gendry and Hot Pie. Interested on how they managed to escape from Harrenhal, he convinces the children to come with them at a local inn. Later, Sandor Clegane is captured by the Brotherhood, and he reveals the true identity of Arya to them. The Brotherhood tries to convict Sandor of crimes because of his long service to the Lannisters, but know of no evil Sandor had personally done to them. Arya speaks up, and accuses him of killing her friend Mycah. He is placed on trial by combat by Beric Dondarrion. After Clegane kills Beric during the fight, Thoros quickly calls upon the Lord of Light to revive Beric, repeatedly chanting "LORD cast your light upon this man, your servant. Bring him back from death and darkness. His flame has been extinguished, restore it!", and in a few short seconds, Beric is alive and breathing again, his wounds healed as well.
Ser Jorah Mormont mentions the Siege of Pyke and Thoros's flaming sword when reminiscing about his knighthood with Ser Barristan Selmy
Some time later, the Brotherhood is found by Melisandre, who is looking for a blood relative (Gendry) of Stannis Baratheon, in order to be able to create more Shadows. She is surprised to find Thoros among them, and berates him for giving up on his mission to convert King Robert to their religion. Thoros takes her to Beric, where she examines him and realizes that he has been brought back from death. When she asks Thoros how many times he brought him back, he responds with six. Stunned, she claims that he should not have that kind of power, to which he simply remarks that he has no power, he only asks the Lord for favors, and the Lord responds. Thoros then confesses how he had always had a large lack of faith in the Lord of Light and never took his duties seriously because of them, until the day that Gregor Clegane killed Beric, his friend, and Thoros was called upon him to revive him. When he saw that it worked, Thoros's faith was restored. Melisandre convinces Beric and Thoros to give her Gendry in exchange for gold, and because the Lord of Light requires him.
Arya Stark could not forgive Thoros and Beric for selling Gendry to Melisandre. Later, during her travels with Sandor Clegane, she includes their names when reciting her "death list".
Remembering how Thoros revived Beric six times, Melisandre attempts a ritual to resurrect Jon Snow following his murder, which is successful.
Thoros helps Beric to hang Lem and other two men for raiding a religious community and slaughtering its people, slandering the Brotherhood's reputation. Sandor Clegane arrives, having hunted Lem down for the attack, and demands to be allowed to kill the men himself. When Thoros sees Sandor he asks "the fuck you doing here?", mirroring the Hound's earlier words to him. When he is given the permission to kill the men, Sandor moves to use his axe, but is stopped by Thoros, who says that the Brotherhood are not butchers - the men must be hanged. He later tells the Hound that Beric and Sandor have both survived for a reason.
Thoros, along with the other members of the Brotherhood, stop at a hut which Sandor stopped in a few years ago with Arya. There, he shows Sandor the power of R'hllor, when he witnesses a vision about The Wall, Wights and Eastwatch-by-the-Sea.
He later helps Sandor bury the previous residents of the hut, a Farmer and his daughter, Sally.
Thoros gets taken prisoner along with Beric and Sandor Clegane by the wildlings guarding the Eastwatch castle on behalf of the Night's Watch. Jon frees them to join the excursion team going north to capture a wight. On the journey, Ser Jorah Mormont comments on Thoros at the Siege of Pyke, but Thoros explained he actually couldn't remember the battle himself. Jorah stated he had thought the red priest was the bravest man he'd ever seen, to which Thoros responded, "or the drunkest".
Shortly after the group finally reaches the mountain peak the Hound had seen in the fire, one of the group's scouts sees a large figure in the distance, much larger than a human. When the creature charges after noticing, it is quickly revealed to be a wight snow bear, which makes short work of the scout. Beric and Thoros both light their blades aflame, and the entire group engages. In the course of fighting the beast, another in the group is killed, and Thoros is gravely injured, having severe lacerations on his stomach and chest. Beric uses the fire on his sword to cauterize the wounds.
They continue to make their way north until they run into an advance patrol of wights, led by a White Walker. Jon engages the leader, killing it with Longclaw. All but one of the wights shatters. They manage to bind the sole remaining wight and carry it when they are forced to flee to a stone island in the middle of a frozen lake. One of the expedition is overtaken by the running wights and pulled under the ice. The cracking ice expands as more dead advance and fall in. Soon the entire island is encircled by a line of broken ice, so the dead are stopped. The expedition is forced to remain overnight, and it is during this time that Thoros freezes to death. At Jon's instruction, Beric burns the corpse with his flaming sword to prevent the red priest from returning as a wight.
A native of the Free City of Myr and a Red Priest for the Lord of Light Thoros was dispatched to the Seven Kingdoms to convert the Westerosi king. Fond of drink, he soon abandoned his faith and became renowned as a tourney fighter, warrior, and carouser. He joined Beric Dondarrion on his mission to arrest Gregor Clegane and became Beric’s right-hand man in the Brotherhood. His faith in the Lord of Light was renewed when he was able to resurrect Beric - multiple times. He encouraged Sandor “The Hound” Clegane to travel with the Brotherhood and journeyed with the group beyond the Wall to help Jon Snow. Thoros was killed by a wight bear.