Ser Gregor Clegane, head of House Clegane and Sandor Clegane's older brother, is a retainer to House Lannister. Gregor is described as being very large (nearly 8 feet (2.4 m) tall and weighing over 30 stone) and possessing enormous strength. He is nicknamed "The Mountain That Rides" or simply "The Mountain". His size and strength make him very powerful warrior and he has earned a reputation of cruelty and brutality.

When Tyrion Lannister demanded trial by combat to prove his innocence in the murder of Joffrey Baratheon, Cersei (Tyrion's accuser) named Gregor her champion. Oberyn Martell, whoes sister was murdered by Gregor, saw a chance to take vengeance and became Tyrion's champion. Gregor won the duel and killed Oberyn, but was poisoned during the fight. Oberyn's poison resulted in a long, agonizing death. Insensible and dying, he was brought to the medical experiment dungeons below King's Landing by Qyburn and has not been seen since. However, the story makes mention of Qyburn's engineering an unbeatable "champion" for Cersei, and Qyburn orders a breastplate too large for any man known to live in Westeros.

From "A Song of Ice and Fire" by George R R Martin