Ser Gregor Clegane is the Knight of Clegane's Keep and is the head of House Clegane, which are landed knights and bannermen to House Lannister. Gregor is a freakishly large man and for this he is often called The Mountain That Rides or simply The Mountain. His soldiers are known as the Mountain's men.
Gregor's history is dark and filled with blood. As a young boy, eleven or twelve years old, he caused his brother Sandor's facial scars by holding the younger boy's face to a hot brazier as punishment for playing with a toy he had discarded. Their father spread the story that the injuries were caused by bedding that had caught fire.
Four years later, Gregor was knighted by Prince Rhaegar Targaryen. There were rumors that Gregor killed his father, sister, and his first two wives. His keep is said to be a grim place where servants vanish unaccountably and even the dogs are afraid to enter the halls. Gregor has committed rapes and cold blooded murders beyond count but his most infamous deed came from his service to House Lannister during Robert's Rebellion.
At the age of seventeen, Gregor was one of the first Lannister soldiers to enter King's Landing during its sack at the end of the war. He and Ser Amory Lorch scaled Maegor's Holdfast when the Lannister troops reached the Red Keep. Gregor entered the nursery of the infant Prince Aegon, son of Prince Rhaegar, and killed the baby present by dashing the boy's head against a wall. Then he raped and murdered Aegon's mother, Princess Elia, supposedly with the boy's blood and brains still on his hands.
War of the Five Kings
During the War of the Five Kings, Gregor Clegane was sent into the Riverlands to raid and pillage and force the lords of the region to turn their eyes inward, rather than mustering at Riverrun. This was a task he was well suited for, and he did it for months as the war raged on. Eventually he rejoined the host of Tywin Lannister, recalled to join the attempt to break the Siege of King's Landing.
After Lord Tywin was slain, and the army broken, Gregor was captured through much loss of life, as it took nearly a dozen men to bring him down, despite the wounds he had taken. He was executed upon the field, and his severed head was sent to Dorne as a means of appeasement.