<P> Gregor Clegane, nicknamed "The Mountain That Rides" or simply "The Mountain", is a fictional character in the A Song of Ice and Fire series of fantasy novels by American author George R.R. Martin, and its television adaptation Game of Thrones . In the books, the character is initially introduced in 1996's A Game of Thrones . He subsequently appeared in A Clash of Kings (1998), A Storm of Swords (2000) and in A Dance with Dragons (2011). </P> <P> An infamous knight and retainer to House Lannister, he is well known for his size, prowess in battle, extremely cruel nature, and uncontrollable temper . He is also the older brother of Sandor "The Hound" Clegane; the two are mortal enemies ever since Gregor gruesomely scarred Sandor by shoving his face into a brazier when they were children . He later becomes the personal bodyguard of Cersei Lannister under the name Robert Strong . </P> <P> In the HBO television adaptation, Clegane was originally portrayed by Australian actor Conan Stevens in season one, and by Welsh actor Ian Whyte in season two; Icelandic actor and strongman Hafþór Júlíus Björnsson took over the role from season four onwards . </P> <P> Ser Gregor Clegane, known as the Mountain That Rides, or simply the Mountain, is the older brother of Sandor Clegane and is a vassal to Tywin Lannister . His size and strength make him a fearsome warrior, and he has earned a reputation for brutality . When they were children, Gregor shoved Sandor's face into a brazier for using his toy, gruesomely scarring him . Over the course of his service to House Lannister, he has committed numerous war crimes, but he is most infamous for the rape and murder of the Targaryen royal family . Clegane was one of the first Lannister soldiers to enter King's Landing during its sack at the end of Robert Baratheon's rebellion . He raped and murdered Princess Elia, and murdered her children Rhaenys and Aegon, a grudge House Martell still bears at the onset of A Game of Thrones . </P>

How many actors have played the mountain in game of thrones
find me the text answering this question