Over its first three albums (if you include the "remix" disc Disguised Masters), Arcturus established a pattern of radically changing its sound each time, a trend that continues with The Sham Mirrors, the band's third "official" studio album.

The constant thread throughout all the group's work, and again here, comes from keyboardist/songwriter Steiner "Sverd" Johnsen, whose sinister, often carnival-esque harmonies and dramatic synth arrangements have a pretty clear stamp by now.

Beyond that, though, comparisons to other Arcturus albums are difficult.

For one thing, the drums and guitars are heavier and more forceful than ever before, and there is more of a traditional metal foundation on this album than on its predecessor, the bizarre, operatic La Masquerade Infernale.

This much is clear from the first shuffling, triplet-based beats of the album opener, "Kinetic." But that same song also shows the band's experimental tendencies in full force, as it quickly derails into a computer-altered, blipping and bleeping interlude before settling into a soaring, smoothly sung vocal refrain reminiscent of Angel Dust-era Faith No More.

The second song, "Nightmare Heaven," throws another curve, as it moves abruptly into a distorted trip-hop breakdown (with Sverd's trademark keyboard harmonies providing the dark ambience) before moving back into dramatic metal territory.

The surprises continue throughout, including a guest lead vocal appearance on "Radical Cut" by Ihsahn of Emperor, whose more traditional black metal screams contrast with all the other vocals on this album.

(Lead throat Garm does everything here from deep-voiced pontificating to falsettos, from spoken whispering to Mike Patton-evoking effects trickery, but he doesn't scream.) A couple of spots on this album feel overly ambitious or perhaps willfully difficult -- "Collapse Generation" feels inconclusive, especially following the disconnected, two-part "Ad Absurdum" -- but the band mostly does a fine job balancing all the unexpected twists and convoluted song structures with memorable, moving melodies (see "Kinetic" and "Star-Crossed" for prime examples).

The Sham Mirrors, even more than La Masquerade Infernale, may end up going down asArcturus' "difficult" album, but it is worth the effort, as this music is unlike anything else being made in metal (or elsewhere) at the time.