After the big success of "Paranoid" Black Sabbath was on tour for almost a year, they spent almost every day together and hadn’t have many days off in 1970 and 1971. In January the band started to record a new album and because the previous album was a big success the record company agreed to spend more money on Black Sabbath’s third studio album. For example the band could use a 24-track board, whereas "Paranoid" was done with an eight-track board. The band also needed more times in the studio to put the album together but they were working hard, according to drummer Bill Ward “Master of Reality” was the band’s peak and they worked the hardest they ever did. The band also decided to change some things: Tony Iommi tuned his guitar down because he liked the heavy sound of a down tuned guitar but the down tuning made it easier for him to play the guitar: The down tuned guitar had reduced string tension and this made it less painful for Iommi, who had two of his fingers partially severed in a working accident some years earlier.
Geezer Butler also tuned his bass down, so that his bass playing fitted to Iommi’s guitar. And the band also slowed things a little bit down and these changes resulted in a heavier sound; “Master of Reality” is probably the heaviest Sabbath album of all times. The album was a success, not as successful as “Paranoid” but it was also a very influential record for later bands. Many metal musicians mention this album as a big influence and it was also the blueprint for a metal genre called Doom Metal, even though many fans will disagree with me. The album’s best songs are the riff monster “Children of the Grave”, probably the heaviest song ever in 1971, the final “Into the Void” and the opening “Sweet Leaf”. Also “Lord of this World” and “After Forever” are very heavy and also good songs. “Orchid” and “Embryo” are short instrumental interludes and together with the slow psychedelic “Solitude” they are a nice contrast to the five remaining songs, which are, as I said really heavy. (in RateYourMusic)