Simplify the following text for a Grade 9 native English-speaking person.
Original: Klas Magnus RosÃ n 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 .
Simplified: Klas Magnus RosÃ n is a Swedish musician , born 1963 in Gothenburg , and was the bassist of Swedish band HammerFall .