Ser Gregor Clegane , popularly known as the Mountain That Rides or simply the Mountain , was a knight of House Clegane in service to House Lannister , the older brother of Sandor Clegane , and a notoriously fearsome, extremely lethal, and much-feared warrior, with a tendency toward extreme and excessive violence.
When Sandor and Gregor were children, Gregor held his brother's face in a fire for playing with one of his toys without permission, horrifically scarring him. Gregor was monstrously huge, he was the largest, strongest, and most feared man in Westeros. His monstrous strength and size were unmatched, and he was one of the most powerful and dangerous people in the Seven Kingdoms.
He was feared throughout Westeros with a reputation for ferocity, brutality, and anger. He has committed unspeakable acts of violence, such as brutally torturing his victims to death, viciously raping women, and murdering children. He was a fiercely loyal retainer and servant of House Lannister , particularly to Lord Tywin. Numerous servants have gone missing at the Clegane keep over the years, apparently killed by Gregor for minor slights in one of his infamous fits of rage, and their deaths then covered up.
Gregor's own father and sister died under dubious circumstances as well, and Sandor thinks that Gregor may have killed them too. Their sister died when Sandor was too young to remember her well, and their father died years later in what was said to be a "hunting accident.
He killed baby Aegon by bashing his head against the wall, and then while still covered in the gore from her children, proceeded to viciously torture and rape Elia. After torturing and raping her for several hours, he then horrifically killed her.
