Simplify the following text for a Grade 6 native English-speaking person.
Original: Halo 2 has sold over 8 million copies in the US making it the best selling original Xbox game of all time .
Simplified: Halo 2 is the best-selling first-generation Xbox game with 8.46 million copies sold by November 2008 . As of September 25 , 2007 , Halo 2 was the fifth best-selling video game in the United States with 6.3 million copies sold , according to the NPD Group .