Black Sabbath, led by Tony Iommi and Ozzy Osbourne, is widely credited with inventing heavy metal. The band's dark, heavy sound was partially the result of an accident: Iommi lost the tips of two fingers in a factory accident and had to downtune his guitar strings to make them easier to play, creating a deep, menacing tone. Their self-titled debut album (1970) and 'Paranoid' introduced themes of horror, social isolation, and occultism to rock music. The use of the 'tritone' or 'the Devil's interval' in their song 'Black Sabbath' cemented their dark reputation. While critics initially hated them, they built a massive fan base and laid the groundwork for every subgenre of metal. Their influence remains the cornerstone of heavy music, and Tony Iommi is revered as the 'King of the Riff.'