Complex sentence: Geezer Butler temporarily left the band in September 1979 for personal reasons, and, according to Dio, the band initially hired Craig Gruber (with whom Dio had previously played while in Elf) on bass to assist with writing the new album.
Split into simpler sentences: Geezer Butler temporarily left the band in September 1979 for personal reasons. According to Dio, the band initially hired Craig Gruber (with whom Dio had previously played while in Elf) on bass to assist with writing the new album.