Gregor Clegane is the head of House Clegane, which serves the Lannisters. He is a monstrously large and fearsome warrior. He is known for being cruel and freakishly strong.
Gregor Clegane is known as “The Mountain” because of his size and strength. He is the older brother of Sandor Clegane, also known as “The Hound”. When they were children, Gregor Clegane held Sandor’s face to the fire and burned half of his face, just because Sandor had played with his toy.
During Robert’s Rebellion, at Tywin’s command, Gregor Clegane killed the Princess Elia, the wife of Prince Rhaegar. He admitted in Season 4 to killing Elia’s children, raping Elia and then killing her by smashing her head.
Gregor Clegane represented Cersei during Tyrion’s trial by combat. He faced Oberyn Martell, the Red Viper. The battle ended with both champions dead.
In season 4, Gregor Clegane is being played by actor Hafþór Júlíus Björnsson.