Complex sentence: Slayer approached the record company regarding a box set release, and discussions lasted roughly a year, due to the fact that the band had to negotiate with the record company concerning plans they had with the box set which were not in their contract.
Split into simpler sentences: Slayer approached the record company regarding a box set release, and talks lasted roughly a year. This was due to the fact they had to negotiate with the record company concerning plans they had with the box set which were not in their contract.