After nine long nights hanging on the great tree Yggdrasil wounded by a spear, sacrificed to himself, the Norse god Odin took up the runes, screaming.

The mysteries of the runes is central to the work of Enslaved, one of the originators of Norwegian black metal, or more specifically Viking metal. 2016 sees them celebrating their twenty-fifth anniversary. Throughout their thirteen-album career they have explored the history, mythology and spirituality of their ancestral home, using it as a prism to view the modern world and explore the inner self: "Seek and find, but do not try and understand" as they put it in 'Roots of the Mountain'.

This great endeavour began in 1991 in the remote village of Haugesund, halfway between Bergen and Stavanger, when Ivar Bjørnson and Grutle Kjellson formed Enslaved out of the death metal band Phobia. They were both very young. Bjørnson was thirteen and had only been ten when a curious Kjellson dropped in on a rehearsal: "I could see the surprise and a bit of disappointment when they saw this 10-year-old kid come out with my chewing gum," Bjørnson remembers now.

They were keenly ambitious and enlisted Trym Torson, another wunderkind, to play drums. They released a couple of demos and then a split EP with Emperor, their side called 'Hordanes Land'. Theirs and Emperor's paths to fame were different but entwined. Emperor were an overnight sensation whose first album In The Nightside Eclipse was released in 1994, the same year as Enslaved's debut Vikingligr Veldi. Both are key artefacts in the development of 'second wave' black metal. Thorson would leave Enslaved after their second album to join Emperor, adding nitroglycerine to their grandiose compositions. But ultimately Emperor flew too close to the sun and split in 2001, at the point when Enslaved were transfigured into the most remarkable band in the genre.

Vikingligr Veldi has recently been re-released, and for the first time on vinyl, completing a cycle of renewal of some of the band's oldest material that still speaks directly to them today: "where we are now feels quite parallel in a sense to those first years: [there's a] clarity of where to take the band onwards," Bjørnson says. “A lot of the things that were done then make sense in light of where we ended up now."

That place is one of the great Earldoms of black metal, built on the boundary-less musical journey the band has taken: from the algific fury of Frost (1994); through the berzerkers-on-acid Ruun (2006); to the transcendent In Times (2015), which sees them both touch the sky and ravage the earth.

Bjørnson stresses that Norwegian black metal in the early 90s was heavily indebted to the work being done elsewhere in Europe, particularly by Master's Hammer from the Czech Republic and Tormentor from Hungary: "we definitely didn't invent gunpowder; I think we found a new way of weaponizing it," he says now.

Enslaved's music was isolated itself within the scene in terms of themes and influences. Bathory provided a precedent with their Viking-era albums (Blood Fire Death, Hammerheart and Twilight Of The Gods) and Bjørnson wanted a level of seriousness that he felt the old Norse ways deserved in culture. It was not about look, as much as feel (enjoyable as the dress-up on their album covers for Eld and Bloodhemn is).

What Vikingligr Veldi lays out is a map of the territory that the band would go on to explore. The title translates as 'Viking Warrior Square' or more loosely as 'The Kingdom of the Vikings', the album "a description of a blank geographical area" being surveyed from above, according to Bjørnson – an epic, cinematic overview, but without the people: they would follow in subsequent albums.

The lyrics are mostly in Icelandic, a language which bears a lot in common with the old Norse tongue and would make the portrayal of a world that existed a thousand years ago more alive. Describing as it does huge swathes of territory, the songs are long and consist of several key musical themes that undulate and warp around "verbalised and concrete imagery". The music is describing awe-inspiring landscapes in an ancient topography.

On opening track 'Lifandi lif undir hamri' ('Living life under the hammer') Bjørnson's first riff is nimble and lithe, dancing with the spooked-out synths, and throughout the song there are stately sub-chords like the Gjallarhorn ringing out across the mountains. Immediately Kjellson's vocals strike you: not drenched in reverb like some of his peers, but somewhere near and full of threat, unseen. His feral, vomitous delivery is immediately one of the most distinctive in black metal, full of character and phlegmy venom. His bass playing is excellent too: clearly distinguishable, he weaves around the majestic slow section of the song. He is even more virtuosic on 'Vetrarnótt' ('Winternight'), making intriguing runs under stunning speed riffing in the second half that plays out not unlike Emperor's 'I Am The Black Wizards'. Trym's blasting gives the song an energy that blazes through the northern sky, a blitzkrieg he also hammers home on 'Heimdallr', concerning Heimdall, the Norse god who personifies Yggdrasil itself. This is not primal music though: it is layered and considered, culminating in the dreamlike washes of 'Norvegr' ('Norway'), delineating its mountains and fjords in shimmering grandeur.

The world of the Sagas in the ninth and tenth centuries concerns their characters' exhaustive genealogies, the currents of power that run between families and the internecine conflicts within them. What is of utmost value is honour as arbitrated by the law – if your relation killed or injured someone, the matter was settled at The Thing (an annual meeting of the great Icelandic families). The settlement was often monetary but sometimes payment was made in blood.

There are stark parallels to the bloody conflict that arose amongst key members of Norway's black metal scene in 1993, which left no-one involved with much in the way of honour. Vikingligr Veldi was put out by Deathlike Silence Productions, a label run by Øystein Aarseth aka Euronymous, the guitarist of Mayhem, who was brutally murdered by Burzum's Varg Vikernes on 10th August 1993. It was released posthumously and dedicated to him.

Bjørnson's recollection of Euronymous paints him as a wandering teacher, travelling around with his guitar showing the bands on his label new techniques and playing them records. He introduced Enslaved to the forefathers of sequencer music like Klaus Schulze and Brian Eno, who inspired a soundbed of synths on Vikingligr Veldi and even space echo effects that can be heard bubbling away on 'Midgards eldar'('Fires of Midgard'): "He was obsessed with that kind of stuff. He had Conrad Schnitzler do the introduction for the Deathcrush album and so on; that German sort of modernism, minimalism and electronica, and Krautrock (a band like Can), was something he was heavily into," Bjørnson says. “He tried to push that onto as many people as he could and a lot didn't stick. Emperor were more into the symphonic stuff and Immortal and Gorgoroth were more into a rock & roll approach to the whole thing. But with us it really stuck, we really went for that and started studying and collecting those albums."

The vision Euronymous held for black metal was that it was inherently progressive music. He was instrumental in broadening how these teenage musicians thought about the guitar. Rather than using two or three strings and perfunctory power chords, he imported traits like glissando from classical music and the grand diminished chords, chromatic scales and layers of harmony that summoned the unmistakable atmosphere of black metal.

"That's where he saw black metal or extreme metal, as a continuation of these, not as a new punk thing where we were wading in blood and pissing on people's heads, but more of an impressionist black and white," Bjørnson remembers. “A gothic cathedral is the picture that he wanted to paint with music - like the Nosferatu movies before vampires became sexy. Where things are cold and derived from human emotion but contain the larger energies."

This points to a bifurcation in black metal: between the progressives and those that see it as a mutation of punk rock. Euronymous' murder was a devastating and senseless blow in these young people's lives.

"We weren't part of that inner black metal satanic thing, whatever they had going on there, but we were close friends, saw him as a musical mentor, both as a band and personally,” says Bjørnson. “When that whole murder business went down it wasn't surprising because there were signs that things were going out of hand. It was of course very saddening but also very frightening because of the guy who did it [Vikernes] and his far from stable friends. He had a lot of friends living in Bergen where we lived, so it was quite a surreal thing, having to deal with the passing of a friend and then at the same time you're 16-or-17-years-old and sleeping with a knife under your pillow at your parents' home. It was weird and scary and at the same time it took away a lot of my patience for the theatrical [elements of the black metal scene] that doesn't do anything except that if a mentally unstable person gets into them it can really destroy things."

Putting on corpse paint and committing outrageous acts confuses action and result. Bjørnson argues that the reason the atmosphere of black metal of the early 90s is so difficult to reproduce isn't because there are fewer unhinged people being born but that often less time is being spent on creating interesting work: "Back then, music and the art were so meaningful to the people creating it,” he says “This image and mystery and holy or unholy air around it was a result of the art and not the other way round. It's an alchemy thing: you can't decide on making artistic magic by reproducing how things looked."

There lies a tension in the primary sources of the Icelandic Sagas about the impact of Christianity versus the anti-Christian mentality of most black metal. A passage from Njal's Saga, written in the late thirteenth century, describes the adoption of Christianity when Olaf Tryggvason was king of Norway between 995 and 1000. The narrator relates that many thought it was absurd to reject the old faith but Njal's view differs: "'It seems to me that this new faith is much better, and that he who accepts it will be happy. If the men who preach this religion come out here, I will speak in favour of it.'" The period of history that followed was one of transition and coexistence of old and new. Thor wasn't known for his seafaring skills, but now the Vikings could turn to a decent sailor like Jesus to get them across the water.

Enslaved's music embraces the darkness but it also celebrates the light. Everything in life has its counterpart: "the beauty in the decline and the rotting and the dying, but that's also leading up to fertile soil, new seeds and birth, and the more tender and mellow sides to life," as Bjørnson puts it. If Enslaved reject monotheistic religions it is because they constrain life's possibilities into a strict code. The concept of Ragnarök itself is misunderstood as the twilight of the gods and confused with the Christian notion of an apocalypse rather than "a point in a cyclical movement that initiates another cyclical movement".

According to Bjørnson, Enslaved's relentless thirst for progression is not about morbidity but the opposite: "a calmness, a loosening, getting rid of that fear that's keeping progress at bay. Progress demands things to end in order to give room to new things."

Also being released this winter is The Sleeping Gods - Thorn, a collection of two EPs recorded at different points. Thorn harks back to the 'necro' beginnings of black metal, its two songs ('Disintegrator' and 'Striker') written on the synthesizer by Bjørnson and subsequently tracked by the band in a cabin in the woods near a plague cemetery south of Bergen. The original cover of the 7" was a black and white illustration of the Thurisaz rune, often symbolising a male, warlike aggression, packed with the symbols of the world religions. It could be interpreted as a confrontational image, but Bjørnson elucidates its more magical properties: "It's a rune with the horns that in rune magic symbolize the act of dimensional penetration. You used it as a fork or a knife to penetrate this veil that is between – in occult terminology that lies between will and act, or reality. And they were all stuffed in there to symbolize that the individual will has maybe a stronger chance of attaining of what one is looking for than all these theatrical, political so-called religions."

The Sleeping Gods came after the "musical high" of the Axioma Ethica Odini album in 2010, tours with Opeth and Dimmu Borgir, and was released by Scion A/V as a free download in 2011. Scion was a sub-division of Toyota (now absorbed into the parent company) and for a few years was run by hipsters who made a point of funding free releases including Sleep, Meshuggah and the Melvins, to whom the rights reverted after a year.

This echoed the industrial era of a philanthropic local factory owner paying for local artists' lodgings and so on. Enslaved had been vocal about the evils of the dissemination of free music online, and even made a point of bizarrely 'stealing' the sheep of a Norwegian politician advocating the legalization of free downloading. But as long as the artist has control and deigns to make it free, Enslaved thought this was the right moment to gift some songs to their fans.

The Sleeping Gods is something of a mixed bag and begins with two songs actually started in 1991, for which Bjørnson had opening riffs but then dissolved to nothing and "were too advanced for where we were musically". As such, 'Heimvegen' starts as NWOBHM homage and 'Alu Misyrki' openly lifts the main riff from Bathory's 'War' before taking flight in the latter mode of the band. Instrumental 'Nordlys' points in the direction of Fields of the Nephilim – "I hate to call it goth rock, it's more guys-in-dusty-leather-coats rock" – and 'Synthesis' is a heavily 70s-ambient track, harking back to the teachings of Euronymous. 'The Sleeping Gods' itself reaches further, to heathen folk that could have been struck up in the halls of the Njalssons themselves.

In the traditions of the Norsemen it was customary for young adults coming of age to depart their homeland and explore the outer reaches of their known world – encountering, plundering, making alliances – in order to return as men. I have to ask Bjørnson about an album that saw Enslaved let the line out from their black metal moorings and take their music into space: Monumension.

The peak of the most problematic period in the band's history, it's a strange, often glorious record which sees them stretching themselves and the affordances of extreme metal, sometimes past breaking point. The heart of the struggle lay in what was seen as an insurgency by then-guitarist Roy Kronheim to imbue the music with a stoner rock edge (the final breakdown riff of 'Vision: Sphere of the Elements – A Monument Part II' being exhibit A for the prosecution).

Where parts of the album fall down as naked Roger Waters worship, there is transformative work here: the astral 'Convoys To Nothingness', the righteous, acerbic 'The Voices' and the monolithic doom of 'The Cromlech Gate'.

It was released in November 2001, a month after Emperor's final album Prometheus: The Discipline Of Fire And Demise, a bombastic conflagration for their old friends. Monumension, though, is a gateway: the band's personal Ragnarök before the albums that followed. The band were out in the cold, "on a mountain, in a cabin, and the electricity's gone, and there's a storm and no food and that kind of shit. And I guess this album is a description of that moment before it knocks on the door and everything's good."

Through that door was the most fertile and consistent run of albums in the band's history, enabled with the recruitment of the members that complete the current line-up: Arve Isdal aka 'Ice Dale' on guitar; Herbrand Larsen on keys and Cato Bekkevold on drums. Anyone who has seen Enslaved live will know that Ice Dale brings a swaggering, shirtless hard rock insouciance to the band that might feel ludicrous if he wasn't such a ripping player. Larsen's importance to their heightened melodic sensibility and clean vocal development is undeniable, his high water marks being 2012's sublime RIITIIR and the inter-dimensional In Times.

Enslaved will perform Vikingligr Veldi in its entirety at the Beyond The Gates festival in Bergen in August 2017, alongside Mayhem running through Euronymous' masterpiece De Mysteriis Dom Sathanas.

Bjørnson has called nostalgia "a sword with at least two edges". The gig could be seen as yet another cash-in of today's retromaniac music culture. But Enslaved are not a band to revisit the past for its own sake: they use it as an older, powerful cycle from which to depart for new lands again. After all, Odin is watching.