[3] The band then relocated to Clearwell Castle in the Forest of Dean, Gloucestershire, England, where guitarist Tony Iommi conceived the main riff of what became the album's title track and lead single.
The band rented a house in Bel Air and began writing in the summer of 1973 but, due in part to substance abuse and fatigue, were unable to complete any songs.
Regarding his writer's block, Iommi admitted to Phil Alexander in 2013, "I panicked because I didn't have a single idea about what to write.
After a month in Los Angeles with no results, the band opted to return to the UK, where they rented Clearwell Castle in the Forest of Dean, Gloucestershire, England, in which the likes of Led Zeppelin, Mott the Hoople and Deep Purple wrote and recorded.
The medieval surroundings may have revitalised the band musically, but also left a sinister impression; in the liner notes to the 1998 live album Reunion, Iommi recalls, "We rehearsed in the armoury there and one night I was walking down the corridor with Ozzy and we saw this figure in a black cloak ... We followed this figure back into the armoury and there was absolutely no one there.
Adds Butler, "We rehearsed in the dungeons and it was really creepy but it had some atmosphere, it conjured up things, and stuff started coming out again".
[4] In 2001 Butler admitted to Dan Epstein of Guitar World, "We almost thought that we were finished as a band ... Once Tony came out with the initial riff for 'Sabbath Bloody Sabbath' we went 'We're baaaack!'"
You'd just lie there with your eyes wide open, expecting an empty suit of armour to walk into your bedroom at any second to shove a dagger up your arse."
Keyboardist Rick Wakeman of the band Yes (who were recording Tales from Topographic Oceans in the next studio) was brought in as a session player, appearing on "Sabbra Cadabra".
"'A National Acrobat' was just me thinking about who selects what sperm gets through to the egg", Butler explained in the liner notes to Reunion in 1998.
In his autobiography Iron Man: My Journey Through Heaven & Hell with Black Sabbath, Iommi reveals that the initial riff for "A National Acrobat" was written by Butler, to which the guitarist "added bits to it.
According to the book How Black Was Our Sabbath, the band invited an orchestra to play on 'Spiral Architect' "but couldn't cram all of the musicians and their instruments into Morgan Studios.
They ended up at the nearby Pye Studio along the road, with Ozzy trying to explain what he wanted them to play like some sort of mad conductor.
[7] In his memoir Osbourne states, "I'd written it one night at Bulrush Cottage while I was loaded and fiddling around with a Revox tape machine and my ARP 2600."
"Killing Yourself to Live" was a Butler composition written while he was in hospital for kidney problems caused by heavy drinking.
[7] Illustrator Drew Struzan created coloured pencil drawings for both the front and back album sleeves.
The back shows a more peaceful scene, rendered mostly in blue, with a dying man surrounded by grieving loved ones and sleeping lions.
[9] The Vertigo LP featured a gatefold cover, with the interior showing a color photograph of translucent members of the band standing in a furnished bedroom.
For the first time in their career, the band began to receive favourable reviews in the mainstream press, with Rolling Stone calling the album "an extraordinarily gripping affair", and "nothing less than a complete success".
[15] Later reviewers such as AllMusic's Eduardo Rivadavia cite the album as "a masterpiece, essential to any heavy metal collection", while also displaying "a newfound sense of finesse and maturity".
[17] In the UK, it was the first Black Sabbath album to attain Silver certification (60,000 units sold) by the British Phonographic Industry, achieving this in February 1975.