Like the scampering little Icelandic imps they are, you can see Sigur Rós but you can’t quite catch them.

For almost 20 years, Sigur Rós have invited musical reference points and then defied them — Slowdive, Radiohead and Animal Collective comparisons have all come and gone. Such comparisons come to reflect, more than anything, the roster of who Sigur Rós has influenced, rather than what the group actually sounds like. Truth is, it’s hard to find a avant-art-post-shoegaze-waver these days who doesn’t credit the Icelandic fantasists with something.

It’s hard to explain their immediate appeal, but frontman Jónsi Þór Birgisson plays a major part. He’s a fascinating, unlikely hero with a Thom Yorkeian falsetto, an enviably stylish wardrobe, his own gibberish lyrical language, and an endless array of mournful, theremin-esque tones wrenched out of his guitar using a cello bow. Contrived? Precious? Sure — you say that like it’s a bad thing.

With seven albums (plus several Jónsi solo albums, plus collabs with boyfriend as Jónsi & Alex), five EPs and countless awards between them, the saga of Sigur Rós is as mythical and expansive as their ostensible subject matter. Forming in 1994, Jónsi and crew didn’t hit it big until 1999 sophomore LP Ágætis byrjun, a deft and way off-kilter fusion of shoegaze, post-rock, lush orchestral elements, sampling, electronic ambience and pop hooks. (This fusion, on paper, totally doesn’t work — you have to hear it to get it). Since ’99, Sigur Rós have led a wave of expansive, celestial post-rock that you’d call ‘ethereal’ if that wasn’t such a loaded word.

Now, full disclosure: this writer generally prefers his post-rock in a harder, more desolate vein — Neurosis, Terminal Sound System, Earth, et al. In light of that, recent Sigur Rós developments are quite exciting. In January, Sigur Rós ventured into the neckbearded caverns of Reddit, where a band member, presumed to be Jónsi, stated, “The stuff we are working on now is very different again… more aggressive.”

Media picked up on it quickly. Two months later, the EP Brennisteinn proved that Sigur Rós weren’t kidding. The 8-minute title track is the first single from upcoming LP Kveikur, set for release in June. “Brennisteinn” is a sonic tsunami that washes well past Animal Collective‘s twee reverb-pop territory and into the spectre-haunted, Spector-ish storm vale of Devin Townsend. Follow-up track “Hryggjarsúla” is a clanking, creeping slice of dark ambience that could be mistaken for a lost Swans recording. Only with the final cut “Ofbirta” does the darkness lift, but even this airy shimmer is tinged with sadness and dissonance.

[youtube]http://www.youtube.com/watch?v=Oc6zXSdYXm8[/youtube]

Sigur Rós haven’t sounded this grimy since Von, their sludgy, Mogwai-influenced 1997 debut. And they’ve never looked like this — the frenetic, gruesome zombie-attack(?) video looks like something Nine Inch Nails might’ve come up with in the ’90s. Where the hell did this come from? To hear it from Jónsi, there’s always been a bit of metal in his approach: “I used to grow up with heavy metal, and Iron Maiden, that band, the singer, Bruce Dickinson, he used to scream a lot,” he said in a 2010 interview. Looks like the beast is unleashed.

Sigur Rós have long been a reference point for metal artists like Alcest and Ulver looking to expand their sound — Sigur Rós‘s producer even headed up the latest Alcest album. Maybe it was inevitable that they’d give some love back.

From the sounds of Brennisteinn, Sigur Rós have used metal’s textures to revive their sound without losing themselves to the genre’s sillier tropes. English soft-proggers Porcupine Tree did the same a decade ago and haven’t looked back. We can’t know yet whether Kveikur will represent a similar permanent sea change for Jónsi and crew– but if Brennisteinn is the red sky in the morning, then we’re about due for a thunderstorm.

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