Fan excitement was high, and understandably so, when Black Sabbath unveiled their 10th album Mob Rules on Nov. 4, 1981.

The previous year's Heaven and Hell had exceeded expectations by delivering Sabbath's strongest set of songs since at least 1973's Sabbath Bloody Sabbath. So, to all outward appearances, the new lineup – consisting of erstwhile Rainbow singer Ronnie James Dio, guitarist Tony Iommi, bassist Geezer Butler and drummer Vinny Appice, in place of a departed Bill Ward – had gelled so well musically that recording this new album should go off smoothly.

But that's not what happened. As author Mick Wall explained in his Black Sabbath biography, Symptom of the Universe, Iommi and Dio had worked almost solo on Heaven and Hell. "What nobody would realize until they began their second album together, in early 1981, was how unbalanced the band's creative ecosystem had now become," he wrote. Wall's point being, whereas Ozzy Osbourne had left the bulk of lyric-writing duties to Butler, Dio had every intention of writing his own words (and having a say about the band's music too), so the compromise was to credit all lyrics on Mob Rules to Dio, while giving Butler a co-writer's credit for all the music, even though Iommi remained responsible for most of those.

In any case, the need for such complicated politics to manage gigantic but easily bruised egos set the temperature for the new album, whose title track was actually recorded ahead of and separately from the rest so that it could be included on the soundtrack to the animated movie Heavy Metal, which bombed in theaters but became a cult hit before long.

Once in the studio, Dio, Iommi, Butler and Appice overcame their issues through the music itself, with a little help from keyboardist Geoff Nicholls and a lot of help from top producer Martin Birch (Deep Purple, Iron Maiden and Whitesnake, among countless others).

Sticking to the general formula that worked so well on Heaven and Hell, they filled side one with a relentless headbanger called "Turn Up the Night," a monster groover called "Voodoo," a true doom colossus in "The Sign of the Southern Cross" and an eerie sound collage called "E5150" (which sort of spelled "Evil" with Roman numerals) that found its way into the Heavy Metal movie alongside the title track.

Side two was less formidable and immediately started showing some of the band's "metal fatigue" in the lifeless "Country Girl" and the Led Zeppelin copy "Slipping Away," but rebounding with the great "Falling Off the Edge of the World" (which boasts some of Dio's best Dungeons and Dragons adventures since Rainbow's "Gates of Babylon"), before winding down with "Over and Over." In the end, it was almost like Black Sabbath's Dio era was running out of steam right before fans' eyes.

As it would – following just one more tour behind the new album. After that, the increasingly strained relationships came to a head during the mixing of 1982's Live Evil, at which time Dio and Appice packed up and moved on to start fresh in Dio. In the meantime, former singer-turned-rival Osbourne had overtaken his old band in commercial terms with his second solo set Diary of a Madman, released only days after Mob Rules.

Though Mob Rules had just managed to nip Diary on the U.K. charts, No. 12 vs. No. 14, in the U.S., the tables had turned turned Top 20 vs. Top 30 in Diary's favor, and the distance would only grow in coming years, until Osbourne was lapping the band that had abandoned him in 1979.

For Iommi and Butler, all they could do in 1982 was start again, luring Ward back and securing the services of Deep Purple singer Ian Gillan for 1983's star-crossed Born Again.

Compared to all this and the even more dire career downturns that followed, Mob Rules would come to be recognized as perhaps the final top-tier album recorded by heavy metal's founding fathers – and this, by definition makes it an essential release for any self-respecting head-banger.

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