Complex sentence: Requiem for a Team is a 2009 documentary film chronicling the history of the Seattle SuperSonics, who were a NBA franchise based in Seattle, Washington from 1967 until 2008 that left Seattle after 41 years in 2008 to relocate to Oklahoma City and become the Thunder.
Split into simpler sentences: Requiem for a Team is a 2009 documentary film chronicling the history of the Seattle SuperSonics. The Sonics were a NBA franchise based in Seattle, Washington from 1967 until 2008, but relocated from Seattle to Oklahoma City, where they are now the Thunder.