In late 1990 and early 1991, Black Sabbath was undergoing a significant shift. After a period fronted by Tony Martin, guitarist and original bassist Geezer Butler began working together for the first time in nearly a decade.
Bootleg collections such as The Dehumanizer Demos or Rehearsals 1991-1992 often feature a mix of instrumental jams and rough vocal takes: YouTube·Boots Bloody Boots
Initial writing and demo sessions took place at in Birmingham. The lineup at this early stage was: Tony Iommi: Guitar Geezer Butler: Bass Cozy Powell: Drums Ronnie James Dio: Vocals (joined mid-process) Key Demo Phases and Recordings
The Dehumanizer development process is notable for its different iterations, many of which were captured on tape and later leaked as bootlegs.