BBC 6 Music ushered in Halloween this year with a gloriously autumnal live session by Sunn O))). On her mid-morning prime-time show the next day, Mary Anne Hobbs played a reverberant section of their 30-minute drone metal track Troubled Air, and introduced the band to her large audience as the “overlords of experimental metal”. Using a dizzyingly large backline at Maida Vale studios, they summoned warm waves of overdriven guitar noise, complemented by a heavenly swell of trombone, which crashed up against current hyped playlist hopefuls and canonical heavyweights including Depeche Mode, Missy Elliott and Tom Waits.

The station presented this bit of curation as shocking (“Yes, you read that right!”) and, in fairness, the grimacing and becowled duo of Stephen O’Malley and Greg Anderson aren’t the first names that spring to mind when you think of daytime radio. Their “songs” often stretch to 20 minutes or more, they’re called things like Big Church (Megszentségteleníthetetlenségeskedéseitekért) or A Shaving of the Horn That Speared You, and consist mainly of a tidal wave of cabinet-shaking guitar drone and teeth-vibrating feedback. Yet they are the most influential metal group of the decade.

This unlikely radio act formed around a pair of metalhead friends in Seattle in 1998; their signature sound was sculpted from loud doom-metal riffs played at glacial pace. They boiled heavy metal down to its absolute essence: a skullcrushing riff stretched out to absurdity and beyond.

Very little about the band has changed in the two decades since. Rather, it is the culture around them that has changed, and some of that is due to the group itself. They are the heaviest band this century has seen so far, and like the giant celestial body they share a name with homophonically, they exert a massive amount of gravity on the culture that surrounds them, drawing more into their orbit while radiating giant waves of creative energy back outwards. Put simply, it’s not all that weird to hear them on daytime radio.

Their influence is, in part, down to a change in how heavy metal is viewed outside the genre’s partisan heartlands. You won’t get any consensus from metalheads on whether it’s a good or bad thing, but over the last 15 years this extreme offshoot of heavy rock has gone from being the most reviled youth subculture there has ever been to being ushered in from the cold. (Initially hated by even the most open-minded of critics, it has a history of being targeted by politicians, demonised by the religious right, and dragged through the courts for spurious reasons.) The band are now more prone to play a set in a concert hall at London’s Southbank Centre than appear at Monsters of Rock, and as likely to be interviewed by Artforum as Metal Hammer. This is because metal is now hip, fetishised by intellectuals – who gather under the umbrella of the International Society for Metal Music Studies – and slavered over by streetwear labels such as Vetements and Supreme. Metal has inspired visual artists Jake and Dinos Chapman and Monster Chetwynd, among others, and been strip-mined by design, film, TV and indeed music itself.

The band have been an oblique influence in the world of industrial and avant techno; Samuel Kerridge is just one producer who has talked of his Damascene conversion after watching them play live. The hard-edged celestial psychedelia of Blanck Mass, which follows movie mogul Samuel Goldwyn’s “start with an earthquake and build to a climax” school of dynamics is clearly the work of a Sunn O))) fan. The links between the band and digital dub/dancehall are even clearer: Kevin Martin (The Bug/King Midas Sound) has long been a vocal supporter of the group and recently graced the cover of The Wire alongside O’Malley.

One of composer Jóhann Jóhannsson’s final pieces of work before his death was the drone metal-inspired soundtrack to Mandy (which was produced by Sunn O))) collaborator Randall Dunn). Geoff Barrow of Portishead, another fan, has mined parallel veins in his own soundtrack work, on such scores as Annihilation. Similar things can be said about the soundtrack work of Nick Cave and Warren Ellis and one-time Sunn O))) touring partner Hildur Guðnadóttir, who recently composed for Chernobyl and Joker. In the field of Americana, Marissa Nadler’s turn towards the reverberant country gothic owes a debt to one of Sunn O)))’s more placid moments, The Sinking Belle (Blue Sheep). More recently, their influence can be felt in the Californian folk doom of Chelsea Wolfe, and the austere but beautiful organ music of Anna von Hausswolff.

Facebook Twitter Pinterest The axeman cometh ... Stephen O’Malley. Photograph: Bertrand Guay/AFP via Getty Images

Why have they been so particularly influential? Metal has, for the most part, always been modernist and avant garde – qualities that have rarely been commented on. Critical revulsion – traditionally class-based but also in some cases responding to disastrous dragon-based aesthetics and questionable gender politics – often stood hand-in-hand with metalheads’ stubborn wish to be misunderstood or to antagonise metropolitan tastemakers. During the 1980s, this led to the incredible situation where even the London-based heavy-metal press often seemed to despise or ridicule now-canonical bands such as Venom, Slayer, Napalm Death, Metallica, Death, Autopsy and Carcass while they were blazing through their most important work. Try looking on eBay for issues of Kerrang! magazine from these acts’ innovative phase – you’re much more likely to encounter issues with Rick Springfield, Phil Collins, Prince or Bon Jovi on the cover.

And when the scene made the unbelievably progressive sonic leap forward of second-wave black metal in the early 90s, this was overshadowed, understandably, by the Norwegian scene’s tragic and hateful spiral into murder, suicide, racism and arson. But if the notoriety and difficulty of black metal ensured the mainstream gave it a wide berth, it also created space for it to evolve.

Sunn O)))’s 2005 album Black One is a a full house on the bingo card of “true” progressive black metal: collaborations with US black metal vocalists Malefic and Wrest, sonic and lyrical references to foundational groups such as Bathory, a cover version of Immortal’s Cursed Realms of the Winterdemons and suitably grandiose technique, as when Malefic delivers his vocals from inside a coffin in the back of a hearse. But importantly, it achieved all this while essentially remaining a hip ambient drone recording.

Facebook Twitter Pinterest Attila Csihar of Sunn O))). Photograph: Maria Jefferis/Redferns

By creating something new while paying all-important tribute to the authenticity of metal, they realised they could take huge leaps forward. In 2009, they released Monoliths & Dimensions, a groundbreaking experimental metal album that owed as much to spiritual jazz, fusion and minimalist composition as it did to devil-horn throwing, bullet-belt wearing, amplifier-worshipping tradition. When the band became one of the first heavy metal groups to grace the cover of The Wire magazine, an apoplectic reader wrote in threatening to cancel his subscription if they featured any more “long-haired idiots” on the front. But a change that would add colour, texture and new meaning to the coming decade was now in full swing, and angry jazz fans were no less able to hold back the tide than King Canute. The world was being forced to confront the idea that heavy metal was, in fact, a valid art form.

You can argue chicken and egg over whether Sunn O))) generated this change in perception themselves or merely rode a new wave. It is probably a mix of both. But there has to be more to it than just metal’s newfound respectability, otherwise I could just as well be writing this feature about the profound cultural importance of Pissgrave or Skullshitter.

Sunn O))) have chimed with the times in other important ways. The monk’s habits and dry ice that typify their shows were initially adopted to mask O’Malley and Anderson’s discomfort at playing live to a sometimes antagonistic audience, but they also helped reconfigure the gigs as rituals, further amplifying the potent sense of spirituality that suffuses metal. My feeling is that the success of Sunn O))) is one of innumerable symptoms regarding the tendency of millennials and Gen Z folk to turn away from the militant atheism of Richard Dawkins and Christopher Hitchens and seek meaningful new ritual outside of organised religion. Others include the phenomenal rise in popularity of tarot cards, the rehabilitation of astrology and the rise of the Tumblr witch, all perhaps connected to the existential threat posed by the climate crisis.

Certainly, alternative methods of combating life’s stresses are being sought with an increasing number of people finding relief from anxiety in yoga and meditation. Sunn O))) have become a soundtrack for the former – there are now doom metal yoga classes – and are absolutely perfect for the latter, as far as I’m concerned. While I don’t want to tar them as hippies, Sunn O))) are, to a certain extent, fellow travellers with musicians operating in the realm of the once derided but now popular new age movement, and : the band have always identified as ambient, albeit with the amusing prefix “power”.

The ultimate intention of the duo’s music, and the crux of the live experience, is transcendence: a positive aim. This may not translate immediately into pure pleasure for the neophyte, as there is a lot to get used to: the pressure, the volume, the vibration. Instead, the experience creates what the psychoanalyst Jacques Lacan would have called jouissance – this is music with a BDSM vibe, an experience that could be viewed as uncomfortable until acclimatised to. At its best, the true psychedelic experience is an analogue of psychotherapy: you are encouraged to lean in to something potentially rupturing or even disturbing, in an attempt to achieve deep personal resolution rather than simply mind-scrambling hedonism or entertainment (which, to be fair, the group can provide as well).

This tacit acknowledgement of positivity is where Sunn O))) differ radically from other revered maverick metal acts such as Mayhem and Napalm Death when it comes to mainstream acceptance – the transgressive history of the former and the strident anarchism of the latter aren’t things that one has to wrestle with in order to enjoy Sunn O))). The knowing pun on death metal in the title of Life Metal, the first of two albums the group released this year, tells you all you need to know. Despite clear and longstanding links with the extreme worlds of black metal, power electronics, industrial, sludge metal and doom, Sunn O))) have created a space that now stands beyond any obvious scene signifiers. This zone of pure affect – and what they hope will be a healing experience – is welcome to all.

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It’s not just their positivity that has endeared them to an urban audience of hip new adopters. While not everyone agrees, I don’t consider the band’s masterful riffology to be weak or diluted, but they have also removed two of the highest and most forbidding aesthetic bars that have previously repelled listeners: the ear-baffling speed and the horrifically gruff vocals of most extreme metal.

So, heavy metal becoming respectable was only the start of it. In an age where people crave spiritual meaning, psychological healing and escape from the hellscape of rolling news and the “constant on” of social media in VR or expensive “immersive experiences”, Sunn O))) offer access to a genuinely new, ego-dissolving space that – outside of smoking DMT or taking ayahuasca in a mud hut in the Amazon basin – knocks all of the others into a cocked hat. They have created a numinous, reality-altering antechamber to a different realm. And this space offers one of life’s most rewarding pleasures: after a long period of hearing something that sounded forbidding or blankly uniform, there is a moment of realisation when the sound suddenly comes into sharp, crystalline focus, presenting an unexplored space of fractal depth, emotion and texture. And then, finally, you can pass into a whole new cultural world.