Scaruffi, Piero (2003). A History of Rock Music 1951-2000, pg. 105. "Black Sabbath (2), a highly influential band, further deteriorated the degree of skills required for playing hard-rock, but their distorted and booming riffs, their monster grooves, their martial rhythms, their monotonous singing and their horror themes, that evoked the vision of a futuristic medieval universe, laid the foundations for black metal and doom-metal."
