The evolution of black metal remains among the most astonishing and unpredictable in all of rock.

How else to describe a musical movement where the same artists whose early efforts comprised some of the most primitive and vile sonic mutations ever concocted could develop into highly skilled songwriters and musicians, capable of works of astounding maturity, surprising accessibility, and, most shocking of all, undeniable class -- all within the span of just over a decade! Of course, very few are the bands that actually weathered this radical transition relatively unscathed, and, considering many of them are gone (Emperor) or dabbling in styles other than metal (Ulver, Tiamat, etc.), Norway's Enslaved have emerged as quite possibly the standard-bearers of black metal's incredible progression.

Devoted Viking metal stalwarts at the start (their early lyrics were written exclusively in Norwegian), Enslaved fared well, but not as well as early contemporaries Emperor and Mayhem, during the mid- to late '90s; but come the dawn of the new millennium, they began an inexorable ascent to the top of their genre thanks to courageously progressive albums like Mardraum, Monumension, and, in 2003, the stunning career pinnacle Below the Lights.

The realization that this peak was then extended into the following year's masterful Isa only makes its achievements all the more triumphant.

Very much a logical progression from its predecessor, Isa sees Enslaved delving even deeper into their strangely psychedelic experiments on black metal, deconstructing lengthy adventures such as "Lunar Force" and "Ascension" from the formulaic speedy riffing, percussive blastbeats, and scratch-throated vocals, to mild-mannered and evocative interludes (often carried by well-placed organs or synthesizers) that one would never expect to hear outside of a '70s progressive rock LP.

Sure enough, shorter, less varied creations like the title track and "Violet Dawning" serve almost as simple linkage points in the greater narrative chain -- a narrative that allows one to view Isa as a single, 50-minute song, if one so desires.

And when superior tracks like "Bounded by Allegiance" and the haunting earth-mother anthem "Return to Yggdrasill" stress their hard/soft contrasts, the results are both mesmerizing and transcendent, finally culminating in the tantric-length climax of "Neogenesis." The sort of epic piece many bands might consider worthy of release as an EP, its 12 minutes begin with a pensive acid rock space shanty, suddenly accelerate into a thrash-fueled frenzy of almost comical proportions, and then disembark upon a beautiful solo section to close the album out on an appropriately grand note.

Really a tour de force by any definition, it's no wonder that Isa consistently ranked among the best albums of the year in most specialized metal publications' year-end polls.