Question: An icelandic professional strongman plays a character nicknamed "The Mountain" in what series of novels?
Context: Sean O'Hagan (born 1990) is an Irish professional strongman competitor and the 2015 Ireland's Strongest Man.  O'Hagan's first major success was becoming Ulster's Strongman Man in 2014, aged 24, many years off his prime and the competition's youngest winner to date. Hafþór Júlíus "Thor" Björnsson (] ; also Hafthor Julius Bjornsson, born November 26, 1988) is an Icelandic professional strongman, actor, and former professional basketball player.  He plays Ser Gregor "The Mountain" Clegane in the HBO series "Game of Thrones". 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). Helena Sverrisdóttir (born November 1, 1988) is an Icelandic professional basketball player who currently plays for Haukar of the Icelandic Úrvalsdeild.  Helena has been named the Icelandic Basketball Women Player of the Year eleven times.  She has won the Icelandic championship two times and the Slovakian championship two times. Robert Oberst (born December 20, 1984) is an American professional strongman who was awarded his "Pro Card" at the 2012 Dallas Europa Amateur Strongman Competition.  He is a yearly competitor of the World's Strongest Man including the 2013 World's Strongest Man, 2014 World's Strongest Man, 2015 World's Strongest Man and the 2016 World's Strongest Man competitions. Mike Jenkins (November 3, 1982 – November 28, 2013) was an American professional strongman competitor from Westminster, Maryland.  Jenkins worked as a high school athletics director when not competing as a strongman.  He was a college and professional football player before switching over to strongman competition in 2007.  He won the Maryland's Strongest Man contest in 2007 which qualified him for the North American amateur national strongman championships later that year and he placed sixth in that contest.  He placed second in the North American nationals in 2009. Brian Shaw (born February 26, 1982) is an American professional strongman competitor and winner of the 2011, 2013, 2015 and the 2016 World's Strongest Man competitions.  Shaw is also the first man to have won the Arnold Strongman Classic and the World’s Strongest Man contest in the same calendar year, a feat he accomplished in 2011 and then replicated in 2015. Serhiy Romanchuk (Ukrainian: Сергій Романчук ; born 6 June 1982) is a Ukrainian professional strongman competitor and national champion powerlifter.  He has competed in the 2010 World's Strongest Man and 2010 Europe's Strongest Man contests.  On Dec. 18, 2010 he won the Strongman Champions League event in Kiev, Ukraine. Reuben de Jong is a New Zealand heavyweight kickboxer, mixed martial artist, strongman and professional wrestler of Dutch descent.  He is a two-time professional strongman champion of New Zealand and competed in the K-1 Kings of Oceania in 2006 and 2007.  De Jong is the holder of two national records in the farmer's walk and stone lifting, and the Guinness World Record for running through the most panes of glass in under one minute.  Also, he was signed with WWE under the name of Russell Walker in 2011, but was released the same year. Edward Hall (born 15 January 1988) is an English professional strongman, notable for being the current World's Strongest Man and the only man to deadlift 500 kg under strongman rules.  He has also won on multiple occasions both the UK's Strongest Man and England's Strongest Man titles.  He is the 2017 World's Strongest Man and the current world record holder in the deadlift with straps.
Answer: A Song of Ice and Fire