Gregor Clegane

Gregor Clegane is a recurring character in Game of Thrones. He was originally played by guest star Conan Stevens and débuts in the fourth episode of the first season. Stevens was unable to return for the second season due to scheduling conflicts. He was replaced by Ian Whyte, who had already worked on the series in other roles. Ser Gregor Clegane is a fearsome warrior in the service of House Lannister. He is raiding the Riverlands on orders from his liege lord Tywin Lannister.

Background
Ser Gregor Clegane is the head of House Clegane and the elder brother of Sandor Clegane, whom he despises. He is a landed knight rather than a full lord. Gregor is huge, said to be the largest man in Westeros, almost eight foot in height and heavily muscled. For this reason he is called 'The Mountain That Rides' or, simply, 'The Mountain'. He is feared throughout Westeros with a reputation for ferocity and anger. He is a loyal retainer and servant of House Lannister, particularly to Lord Tywin.

Season 1
Ser Gregor takes part in the tournament to celebrate Eddard Stark's appointment as Hand of the King. He kills Ser Hugh of the Vale in a joust, horrifying the crowd. Littlefinger tells Sansa Stark that Gregor burned his brother Sandor's face over a naked flame when they were boys because Sandor borrowed one of his toys without asking.

Catelyn Stark takes Tyrion Lannister into custody on suspicion of the attempted assassination of her son Bran. In retaliation Gregor is dispatched to raid the Riverlands, her homeland. Refugees from the Riverlands attend court in King's Landing and report that Gregor has sacked their villages. Eddard Stark sends Ser Beric Dondarrion with a hundred men to bring Gregor to justice. Eddard declares him a false knight and is strips him of all ranks and titles, lands and holdings, and sentences him to death.

When Robb Stark's victory against his son forces Lord Tywin to withdraw his forces to Harrenhal, he orders Ser Gregor to take five hundred men and "set the Riverlands on fire" from one end to the other.

In the books
In the Song of Ice and Fire novels Gregor Clegane is a ferocious warrior. He is so large that he wields a two-handed sword in one hand, and wears armor that is so heavy most men cannot even lift it. He can only ride the largest and strongest horses. It is heavily rumoured that Gregor committed horrific and unspeakable deeds during the Sack of King's Landing that ended Robert's Rebellion, including the rape and murder of Princess Elia Martell and the murder of her infant son Aegon Targaryen.

Gregor's father, sister and first two wives all died in suspicious circumstances, leaving Gregor and Sandor as the only living members of their family.