Sigur Rós, now recently diminished to a three-piece with the exit of longtime keyboardist Kjartan Sveinsson, are swiftly hurtling into their twentieth year together. Every record they've sired in those two decades is like a relic of deific proportions; from the fluid atmosphere of () to their breakthrough anthology of orchestral post-rock, Takk..., the Icelandic icons somehow encapsulate the most fragile, mortal fragments of emotion, of relationships with nature and of life with their sound, in a way that most musicians could only dream of. The band are famed for legendary live performances capable of reducing even the burliest sourpusses to tears in the wake of their beauty, standing true as kings of the genre.

After a three year hiatus, the group resurrected themselves with last year's Valtarí; it was a collection of sprawling ambient paeans, vaguely nautical and organic, but vastly subtle and restrained. It was immensely different from its predecessor. At the time of its release they spoke of plans to record even more after Valtarí, which came as something of surprise, given their penchant for sluggish writing processes and sporadic schedules. However, come autumn 2012, bassist Georg Hólm confirmed that indeed, there was a new album in the works. An "anti-Valtarí", as he called it.

Then, in February of this year via XL (the band's new label), 'Brennisteinn' arose from the underworld. Drenched in post-industrial doom, it's more like Rammstein or the more malevolent streak of Aphex Twin than Sigur Rós - the only tangible link is Jonsí's familiar falsetto. Enormous creaking bass lurches and resounding percussion incite more fear than awe - this is not like anything the group have done before, and in the blink of an eye, the band have set alight all memories of them being placid, meditative or calm. This is a self-immolation. It forgoes the usual ice-queen glory and chilled snippets of ethereal rock, instead we've got desolation and flames. Fire and brimstone. It's a fresh facet that remains at the forefront of Kveikur.

Heralded by a eurodance beat, 'Yfirborð' ascends to dizzying heights with warped effects, pounding rhythms and tearjerking vocals; parts of Jonsí's voice are reversed/pitch-shifted/absorbed by effects that it sounds like the death rattle of a once mighty hero. 'Stormur' is piano-led, recalling the isolated fragility of the opening to 'All Alright', before careening into a monolith of resentment and agony - the percussion is fantastic, not since 'Gobbledigook' has Orri (Páll Dýrason, drums) had a time to shine. Potentially the darkest, most menacing moment available on Kveikur - yes, even surpassing 'Brennisteinn' - is the title track. It opens like an anthem of demons: child choirs with Exorcist-level creepiness, more industrial stabs of mechanical bass and goth drums, Jonsí's voice is swallowed whole - he's always able to showcase a breadth of emotions from his vocal chords, but this could be the first time we hear anguish. He hoarsely groans like he's grasping for his last breath.

Most - but not all - of what you can glean from this record is tinted with negativity (sorrow, anger, terror), 'Ísjaki' attempts to defy that. It's the aural equivalent of the girl in red in Schindler's List. Perhaps one of the most aorta-pulverising tracks Sigur Rós have ever fashioned from mere instruments, it's got an overt sense of hope and pop-glazed anthemic optimism. However, it's still a devastating cut when slammed into the ranks of the rest of Kveikur. Despite its positivity, the rest of the album's innate gloom drags it back into the sinister depths - it becomes a monument to what the rest of the album isn't. Starkly contrasted against the backdrop of perfect misery, it's just as soul-sucking to listen to, knowing that it's actually a symbol of dying hope and waning optimism.

Sigur Rós have dabbled across an array of genres, but have always stuck fairly close to their home turf. Tonally and timbre-ly, you can always pick out an effort by them from a line-up; although that's no bad thing, and the reaction they garner anywhere they go proves that they're pleasing a lot of people. Kveikur sees them potentially delve into a different territory for the first time. It's plagued by shadows, complex textures and frantic will-o'-the-wisp rhythms that dart and weave through the music - when Hólm described this as an "anti-Valtarí", he was spot on. It's unsettling, chaotic, vengeful, astounding, invigorating and forces you to feel a plethora of cacophonous emotions concurrently. It's their loudest record. It's their darkest record. It's their best record in a long, long time.