For all its cosmic, world-swallowing bluster, there's something reassuringly traditional about much stoner and doom metal. The dark fires set by Black Sabbath, Saint Vitus, Sleep and other forefathers still smoulder in the sound of many acts, and for the most part it's something to be celebrated.

By the same token, it also makes those who do it exceptionally well stand out from the crowd, and over the past ten years Italian beard-fanciers Ufomammut have been doggedly climbing their way to the threshold of masters with their dense, prog-tinged grooves. Now, with the release of Opus Alter and the completion of their colossal ORO work, they've emphatically crossed it.

Opus Alter proudly takes up the torch from its forebear, Opus Primum to form the second half of ORO, an exploration of 'knowledge and power' the band insists should be approached as one whole experience. And altogether, it works; the two halves complement each other effortlessly, combining to create one enormous, uncompromising but satisfying listening prospect.

Where Primum often worked towards textured heaviness, crushing your ears into willing submission by way of slow accumulation of layers, Alter is somewhat less formal, decked out in the sort of unashamedly bludgeoning, sledgehammer statement riffs to make Mastodon cry. Not to say Primum wasn't already heavy as fuck, but if that first half was the wild eyed stranger who coaxed you into the alley for a toke of his funny-looking cigarette, Alter is the point at which you realise you're smoking a neutron star.

Opener 'Oroborus' exemplifies this, its churning, mammoth bass groove exploding into a monstrous tidal wave of sound that you absolutely.must.headbang.to. The motif rears its head again later in the saga as 'Sulphurdew's Rammstein-a-like stomp mutates back into something more than a little similar.

Ufomammut's purity is a large part of what makes them so appealing – there are no half measures here, each track's singular core given ample space to breath. Nothing is permitted to last less that six minutes, each rumbling riff worming its way into your mind with relentless clout.

There's a focus on structure and dynamics that can be lacking in other riff-mongers, and while there's nothing quite as pleasingly melodic as earlier tracks like 'Lacrimosa', Opus Alter still pushes boundaries as it forges a hypnotic path through waters deep and dim.

Despite its directness, this is still music that is entirely nuanced, always delivering new subtleties on repeated listens and always startling in its ability to plumb new depths of numbing heaviness. To put it bluntly there's a lot going on here at any given moment, and it's been engineered to digest over a long time.

'Sublime' forgoes the pounding immediacy of the rest of the record to build itself up by degrees, harking back to Primums opener 'Empireum'. When it reaches maximum velocity, however, it's gloriously punishing.

Then there's closer 'Deityrant', which slams into your synapses like an enormous, lurching beast galloping across endless plains. It's suitably bombastic for a final movement to close the whole ORO cycle, though ironically manages to feel a little anti-climactic after all the epic graft leading up to it.

Of course, this sort of thing isn't for everyone; Ufomammut have no interest in crossover appeal, and their music is recklessly challenging in its heaviness. Nonetheless, the craft and impact of Opus Alter is something to marvel at, and it will no doubt raise their obsidian star even higher in the stoner firmament.