Klas Magnus Rosén is a Swedish musician, born 1963 in Gothenburg, and is best known for being the bassist of the Swedish band HammerFall during the years 1997-2007. Magnus Rosén started to play bass when he was 15 years old. When he had played bass for three years he went on his first tour with the band Kung Sune. After the tour he moved to Los Angeles, there he played in many bands, like Billionaires Boys Club where future Hammerfall-bandmate Anders Johansson played drums. He became a member of the Swedish heavy metal band Hammerfall in 1997.