The band who laid the foundations of heavy metal are releasing 13, their first studio album with Ozzy Osbourne for 35 years. Black Sabbath speak about drugs, the occult and God

It's hard to know where to start. There's the simple fact of Black Sabbath's return with 13 – the metal overlords' first album of new material with Ozzy Osbourne since 1978's Never Say Die! – which led Kerrang! magazine to call it "rock's biggest comeback in 35 years". There was the 11th-hour decision by original drummer Bill Ward to withdraw from the reunion due to contractual wrangles. There's the soap opera that is Osbourne, who when I speak with him in April rants about overindulgence, only to appear in the tabloids a week later after falling off the wagon. And then there was guitarist Tony Iommi's cancer diagnosis in 2011. Which means it probably makes most sense to begin with "How are you?"

"I'm not so bad," says Iommi, in a London hotel with bassist Terry "Geezer" Butler (Osbourne speaks a few weeks later but for the time being he's had to fly back to Los Angeles where his son Jack is having therapy for MS).

"The treatment [for lymphoma] is ongoing," explains the human riff machine behind rock classics such as Paranoid, War Pigs and Iron Man. "I did ask them: 'Has it gone now?' and they said no. And it won't. You've just got to deal with it. As you get older, you're going to get something, aren't you?"

"It's inevitable, isn't it?" agrees Butler, still, like Iommi, goateed and black-clad after all these years, even if the hair is now merely shoulder length.

Black Sabbath's murkily monolithic metal, which captured the dark late‑60s moment when the hippy dream turned sour, is immortal but its creators are as prey to the ravages of time as everyone else. They laid the foundations for an entire subculture – MTV placed them at No 1 in a list of greatest heavy metal bands while whole generations, from the late-70s new wave of british heavy metal to grunge, have proclaimed them godfathers – but they won't be around for ever.

Then again, they've been having close scrapes since the late-60s. Butler remembers experiencing excruciating kidney problems, believing he was dying. It was then that his parents, strict Catholics who would "whip and beat" him if he cursed ("which," he says, "is why I rarely swear now"), witnessed the full extent of their son's immersion in the dark arts: they recoiled in horror at his black-painted bedroom, with its proliferation of Satanic paraphernalia.

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"The doctor called them to take me to the hospital," he recalls, ever the droll Brummie, "and my dad came in, like, 'Mother of God!', ripping all the upside-down crosses and pictures of Satan off the wall."

According to Butler, who has been largely responsible for their lyrics over the years, when they formed, Black Sabbath never set out to revel in evil but to tap into the malevolent currents coursing through society. The Vietnam war was raging and there were calamities such as the slayings in Los Angeles by Charles Manson and the murder by Hells Angels at the Rolling Stones' Altamont concert. Amorality was in the air and, as the 70s loomed, Black Sabbath caught it.

"War was the main theme [of our songs]," Butler says. "My brothers were all in the army and I thought I'd have to go and fight in Vietnam. Then there was the atomic bomb and the feeling that we were all going to get blown up."

"It was peace and love," chips in Iommi, "but then it all went pear-shaped."Sabbath have always been associated with the sinister, but they became not so much proponents as harbingers of doom after Butler had a vision of the devil in his late teens.

"It was a black shape," he says, "with horns and everything. I freaked out. That's when we started warning against satanism."

Surreally, Butler recalls being on Top of the Pops for Paranoid and having a conversation about the almighty with Cliff Richard.

Black Sabbath in 1970: Iommi, Osbourne, Butler and Bill Ward. Photograph: Chris Walter/WireImage

"I had a Sabbath [upside-down] cross on and he went: 'Oh, are you a Christian?' and I said: 'Yeah.' I used to be very religious," he says, "but I don't really believe in anything now."

Questioned about faith after being confronted with his own mortality, Iommi says he believes in God. Butler, who still wears a crucifix round his neck, adds: "I do when I'm on a plane – the first thing I do is say my prayers." As for Osbourne, who came up with the title for new track God Is Dead?, he is uncertain, even if he does talk with the conviction of a preacher.

"I've done a good job of trying to kill myself for 64 years," he says over the phone from his home in LA. "But something's kept me alive. I don't know if it's God, fate or whatever. I'm open-minded. But I don't believe in organised religion at all."

When I suggest God Is Dead? plays into Sabbath's art of darkness, Osbourne brings up a few myths, including the one about him biting off the head of a live bat, which he had presumed was rubber, in 1982.

"We were never really into the occult," he says. "It was a hobby, until we started getting invites to black-magic rites in cemeteries. Then I got accused of doing this and biting that and there would be people picketing the arena with banners. But if you listen to the lyrics, there's nothing that's pro-black magic or pro-satanic worship. It's like God Is Dead? – at the end it says: 'I don't believe that God is dead', but they only hear the words 'God is dead'."

Maybe there has been a conflation of badness and toughness. Black Sabbath have always seemed hard, resulting from their working-class upbringing in the grim West Midlands.

"It was rough in the early days," says Iommi, who lost the tips of two fingers on his right hand in an accident at a sheet-metal factory. "There would always be punch-ups with somebody – even the women."

Longhairs in a city full of skinheads, Sabbath were outsiders.

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"If we went to any club we'd be outnumbered," says Butler. "At the end of the night we'd get walloped. Tony and Ozzy were handiest. But if anything happened, we'd all dive in and help."

"There were stabbings," adds Iommi, jolting the bassist's memory.

"I saw a guy get stabbed to death outside my mum's house," Butler remembers. "He died in her arms. Saturday night would be absolute murder. That's why we couldn't wait to get out of Aston."

Within three years they were one of the biggest bands in the world. Following the success of Black Sabbath (1970), Paranoid (1970) and Master of Reality (1971), the outsiders found themselves, along with Led Zeppelin and Deep Purple, pioneers of a heavy progressive rock movement. They were even big in the US, where they went to record their fourth album, 1972's Black Sabbath Vol 4. There, the issue wasn't black magic but white powder.

"We were young blokes, doing what young blokes do," says Iommi, referring to their prodigious consumption of cocaine – the album featured a track called Snowblind. Holed up in a Bel Air mansion, their degenerate behaviour, involving substances and groupies, was, decides Butler, worthy of Caligula.

Iommi dismisses the idea that he, Butler and Ward were the stabilising force in the band for the madcap Osbourne. "Nobody could control anyone else," reflects the guitarist, who almost overdosed one night at the Hollywood Bowl. "I was doing coke left, right and centre, and quaaludes, and God knows what else. We used to have [cocaine] flown in by private plane."

Vol 4, says Butler, cost $60,000 – $15,000 less than their cocaine bill. The bassist recalls how someone spiked his drink with acid, causing him to try to "end it" by jumping out of his hotel window. "Tony and Bill had to hold me down on the bed," he shudders. "I started going off drugs after that."

Osbourne had no such plans. His bandmates remember him frequently going awol. Trouble was, Sabbath's monumental sound required focus in the studio, and he couldn't comply.

"You can't write or play songs if you're out of your brains," says Butler. "But because [Osbourne] didn't have to play an instrument, while we were writing he'd be in the bar getting legless or doing all kinds of things." The latter included relentless pleasure‑seeking, leading to blackouts.

"I didn't believe it when they said I had blackouts," says the singer. "I'd look at my watch and it would say four o'clock. Then I'd look again and it would say 9.30. I'd totally forget where I'd been or what I'd done. But then you don't think about that before you start drinking. You don't think: 'I wonder what jail I'll wake up in tomorrow?' Because that's not your intention."

For Osbourne, Vol 4 was Sabbath's peak, after which the band fell victim to infighting and financial mismanagement. "We were like a dying star and we should have pulled the plug there and then," he groans.

According to Butler and Iommi, Osbourne's capacity for self-destruction was so great that if he hadn't left the band in 1978 "he would have died". As it was, says Osbourne: "I ended up losing my mind." If the 70s were a blur, the 80s climaxed in tragicomic style when, in a drunken stupor, he tried to strangle his wife.

"I woke up smashed out of my brains, charged with attempted murder," he reveals. "I was like: 'Who the fuck did I try and kill?' The policeman read the charge sheet: 'Sharon Rachel Osbourne'."

In the past three and a half decades, there have been various Black Sabbath lineups, and Osbourne has also had a successful solo career since the release of 1980's Blizzard of Ozz. There was an attempt in 2001 by the original members to record an album with producer Rick Rubin but it came to nothing. Now, sufficient water has passed under the bridge to make them – minus Ward – eager to make it work. It still felt, Rubin has said, "like being in a cage with a lion, keeping them calm", but the results, they agree, were worth it. 13 sounds more like Black Sabbath than any album since 1973's Sabbath Bloody Sabbath. Osbourne says 13 is the most important record of his career.

"Yes, we've had our disagreements," he says. "We all said bad things and did our different trips, but it's great to be with the guys. The last time [2001] it didn't gel. But this time it worked out better than our wildest expectations. For Tony to come up with riffs like these is unbelievable. He deserves a medal because if I'd been diagnosed with lymphoma I'd have gone 'Fuck this', and gone back to bed with a bottle of morphine. But we kept on going because we knew this was probably our last hurrah."

Now all he's got to do is stay sober.

"With or without alcohol I'm a fucking lunatic. I jump to the dark end of life. Probably," he suggests, "why I'm the singer of Black Sabbath."

• 13 is released on 10 June.