Dylan Carlson’s Earth have been around for so long now - and so frequently imitated - that it can be hard to recall just how radical they sounded when Earth 2 came out in 1993. I was only 11 at the time, but I still remember the impact it had when I discovered it years later, having already ingested much of its legacy. No matter that I already worshipped at the altar of SUNN O))), Boris and Orthodox, Earth 2 remained a revelation: dense, hypnotic and heavy as a ton concrete, even though it was bereft of drums. With guitars ramped up to the nth degree, but tuned to gut-wrenchingly low frequencies, Earth carved out a veritable canyon of pure molten drone, one which would have a profound influence on modern metal music.

And then they did it again! After many years troubled by drug addiction, Carlson took a break in the late-90s, only to return with a revamped Earth in 2005, one that was no less heavy but which allowed for a bit more space, for want of a better term. In came drums, slide guitar and even trombone, opening up room for Carlson’s guitar to roam and lumber, like a hulking bison ploughing across Midwestern plains. Hex, or Printing in the Infernal Method was another triumph, and generated a brand new furrow that the band has been ploughing ever since.

Angels of Darkness, Demons of Light II is very similar to its twin, released last year, in that it shows Earth taking the model forged on Hex and expanding on it in subtle, but fundamental ways. I have to admit that I was rather nonplussed with 2008’s The Bees Made Honey in the Lion’s Skull, which followed the country-doom of Hex but didn’t seem to make any progress on that album. The arrival of bassist Karl Blau and Lori Goldston on cello have totally transformed the band, even as Carlson and drummer Adrienne Davis continue to build on their previous output.

The way Earth go about unfurling their doom-laden tunes is impressive in its deliberate slowness. At times, listening to Angels of Darkness, Demons of Light II made me think of Japanese post-rockers Mono’s 2008 album with World’s End Girlfriend, Palmless Prayer/Mass Murder Refrain, and not just because of the presence of strings. Carlson and his bandmates linger over every riff, every drum and bass pattern and every sweep of the cello, putting emphasis on the dark, haunted character of the music. Earth have always been a "dark" band, but with the cello becoming a ghostly, intangible presence underneath and around the guitars, the atmosphere on Angels... is perhaps even more lugubrious, the tone feeling more suited to a funeral procession under grey skies where on previous albums it had the arid quality of a lonely horseback ride through the desert. This new angle is perhaps best reflected in the album’s artwork, depicting skeletal demons marching to war, an army of terror and spectral beauty.

At the same time, the music on Angels... is unnervingly melodic, with elegant guitar progressions and more supple, elegant drumming from the underrated Davies. When I first heard this album, and its predecessor, I’d just been listening to the elegant jazz of Charles Mingus and Bill Evans, and Earth’s sound seemed to fit neatly alongside those guys. There’s an almost dancing, horn-like quality to Carlson’s motifs, with more pronounced solos. Nowhere is the more perfectly balanced than on 'His Teeth did Brightly Shine', where muted guitar strumming gives Carlson free reign to rumble and roar over the top with his usual restrained aggression. On 'A Multiplicity of Doors', a 13-minute masterpiece, the cello takes prominence, bringing melancholic despondency into the centre of Earth’s rugged metal chug. At times, you get the sense that Earth are looking back into a European folk past, therefore going even further beyond the metal, jazz and alt-country explorations they’re already known for. You can almost hear strains of Fairport Convention circa Liege & Lief, minus the liberation of that album’s more sunshine-y passages. At other times I was reminded of the nightmarish Nosferatu-themed album by Popol Vuh, Bruder des Schattens, Sohne des Licht, a fact perhaps emphasised by the title.

I’m usually a bit wary of artists putting out two tomes of the same material, as it feels like an attempt to hide the fact that they were unable to make tough decisions on what to cut. With Angels of Darkness, Demons of Light, however, it feels more that Earth have given us time to absorb what may come to be seen, in retrospect, as something of a magnum opus. Time will tell if Angels of Darkness, Demons of Light deserves to mentioned in the same breath as Earth 2, but there is no denying that Dylan Carlson and his band are determined to keep pushing at the limits of their singular version of modern metal.