Black Sabbath Vol. 4_ was the first album where classic sludge-rockers Black Sabbath (Ozzy Osbourne-vocals, Bill Ward-drums, Geezer Butler-bass and Toni Iommi-guitar) started experimenting - which possibly foreshadowed what would be more emphasized on the following album, _Sabbath, Bloody Sabbath_. I just want to make a clarification before I move on: Black Sabbath's music is not about satanism or devil worshipping - it's subject matter is mainly about the harsh realities of life (i.e., crime, war, drugs, mental illness and more), which is rather "dark".