Ser Gregor Clegane is the leader of House Clegane, a chivalrous house from the Westerlands, and the elderly brother of Sandor Clegane. When Sandor and Gregor were kids, Gregor held his sibling face in a fire for playing with his toys without consent, horrifically scarring him. Gregor is a landed knight rather than a full-fledged Lord. Gregor is strangely huge, said to be the largest man in Westeros, almost eight feet in height and heavily muscled. He is dreaded throughout Westeros with a notoriety for ferocity and anger. He is a faithful retainer and servant of House Lannister, particularly to Lord Tywin.
Numerous lackeys have gone missing at the Clegane keep over the years, presumably killed by Gregor for small slights in one of his notorious rages, and their losses then hushed up. Gregor’s own father and sister died under suspicious circumstances as well, and Sandor half-suspects that Gregor may have removed them as well. Their sister died when Sandor was too youthful to remember her well, and their father passed years later in what was said it happened in a “hunting accident.” At the direction of Tywin Lannister, Prince Rhaegar Targaryen himself anointed Gregor as a knight of the realm, a great honor indeed, even though Gregor was an infamously despicable man.
During Robert’s Rebellion, one year following Prince Rhaegar Sir him, Gregor engaged in the Sack of King’s Landing. After entering the Red Keep, he brutally killed Rhaegar’s two kids by Elia Martell: their daughter Rhaenys and baby Aegon. He murdered baby Aegon by bashing his head against the wall, and then while still drenched in the gore from her children continued to rape Elia, after which he killed her. It is rumored that he murdered her by cutting her in half with a single swing of his massive sword, though he later declared to have “smashed her head in”.
Despite requests by Ned Stark, neither Gregor nor Tywin were punished by King Robert for the crimes of the royal family, pointing to a rift in Ned and Robert’s friendship.