Gregor Clegane is the head of House Clegane, a house loyal to the Lannisters. At the age of 12, he caused his brother Sandor's facial scars by holding the 7-year old up to a hot brazier for playing with a toy he had discarded. It took three grown men to pull them apart. To explain the boy's scarred face, their father spread a story that his bedding had caught fire. Gregor became head of the household after their father died during a suspicious hunting accident. He is also said to have had a younger sister that died in mysterious circumstances.
He was a squire by the age of twelve, and was knighted by Rhaegar Targaryen at the age of sixteen. During Robert's Rebellion, he was one of the first Lannister soldiers to enter King's Landing while it was being sacked. Gregor entered the nursery at Maegor's Holdfast and killed the infant Aegon by smashing his head against the walls. Afterwards, he raped and murdered Rhaegar's wife Elia Martell.
He is well known for his enormous size, brute strength, and for his cruelty. He is a solitary man, rarely ever leaving his lands except for war or tournaments. He also suffers from constant headaches and consumes vast amounts of milk of the poppy to dull the pain. The Clegane Keep is said to be a grim place where servants vanish and even dogs are afraid to enter the halls.
Daniel Abraham adapted "Game of Thrones" from the fantasy series "A Song of Ice and Fire". Gregor Clegane first appeared in Game of Thrones #9.
Ser Gregor takes part in the Tourney of the Hand, slaying the newly-knighted Ser Hugh of the Vale in his second joust when his lance pierces the boy's throat. He is unhorsed by Loras Tyrell, who rides a mare in heat to unnerve Gregor's stallion. Gregor goes into a rage, killing his stallion in one blow and attacking Ser Loras. His brother Sandor intervenes before Loras can be hurt, and the two duel shortly until King Robert orders the two to stop. Gregor rides to his keep, and is ordered by Tywin Lannister to raid the riverlands after Catelyn Stark kidnapped Tyrion Lannister.
Gregor Clegane first appeared in ''A Game of Thrones'', a novel written by George R. R. Martin. Gregor is one of the main antagonists in the series ''A Song of Ice and Fire''.
Gregor appeared in the HBO television series, based on ''A Song of Ice and Fire'', titled ''Game of Thrones''. He is portrayed by Australian bodybuilder and actor Conan Stevens in the first season, but due to scheduling conflicts, he was replaced by Ian Whyte in season two.
It has been recently announced that Hafþór “Thor” Björnsson will play Gregor Clegane in Season 4.