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".
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 GregorClegane 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.