Genre tags are becoming increasingly superfluous in the metal realm. With metal splintering off into wildly heterogeneous directions, subgenres dissolving into ever more complex subgenres, and bands adding decidedly non-metal weaponry to their arsenals, a simple tag may still be the easiest way to reference elements of a band’s aesthetic, even if it won’t necessarily reflect their entire palette. Take the case of Arkansas-based “sludge” outfit Rwake, whose new album Rest is such a rich sonic banquet that the term sludge is a woefully self-deprecating moniker to describe what’s on offer.

Sludge was born from the bluesy grind of Sabbath fused with a lethargic Southern flavor. Its foremost progenitors—Grief, Eyehategod, and Buzzov*en—laid out a template of quagmire-deep down-tempo and distorted riffs mixed with barking acidic vocals. While those fundamental components still apply to plenty of present-day sludge bands (the magnificent down-home stomp of Weedeater being an excellent example), the genre witnessed a revisionist explosion in the mid- to late-‘90s. Many bands poached those core characteristics before setting off on riotously inventive journeys. Sludge might be a ubiquitous term to define a certain swagger, but with bands as diverse as Boris, Baroness, Pelican, and pack leaders Neurosis all harnessing elements of the genre, you’d be hard pressed to define contemporary sludge as anything straightforward.

It’s been four years since we had any new material from Rwake. The band’s last release, 2007’s Voices of Omens, was justifiably heralded as a fine example of sludgy hallucinogenic escapism. And while Rest only offers up six new tracks, there’s no shortage of innovative ideas—the tracks are stretched out to epic proportions to encompass the band’s overarching vision.

Rwake certainly draws from the basics of sludge, but it’s progressed to a point where any basic descriptor does a real disservice to the kind of forward-thinking and dynamic craftsmanship displayed on Rest. While the band’s southern credentials are secured with a sizeable wall of crushing harmonics, it also incorporates a wide set of influences to build on that swampy foundation. A heady progressive spirit runs through Rest. Hefty, contorted guitar passages, demented, doom-laden vocals, and psychedelic effects are set against a monolithic backline to produce a vicious rant wrapped in some mesmerizing atmospherics.

After the delicate intro of “Souls of the Sky”, the band launches into the 12 mordant minutes of “It Was Beautiful But Now It’s Sour”. Warping labyrinthine riffs, along with a curt little archaic Eastern solo, set the scene before vocalist C.T. begins to wail with corrosive abandon about the “heartache of losing everything you know.” Co-vocalist (and moog and sample provider) B. also makes her first appearance on the track. Her blackened shrieks underscore C.T.‘s apocalyptic narratives as waves of riffs crash over and over till the song dissolves in a dark ambient surge.

The first 15 minutes of Rest reveal a band eager to push at the boundaries. While obviously still happy delivering the mystical eulogies of its past work, Rwake has clearly spent its downtime developing a newfangled almost kosmische orientation. While you’d be hard pressed to draw direct parallels with any Krautrock innovators—Rwake is far too cantankerous for that—there are flourishes of churning motorik rhythms scattered throughout the album, and an almost bohemian hankering to experiment with unorthodox tempos.

A psychedelic pulse has always existed on Rwake’s albums, but the past few years have seen many sludgy bands explore the possibilities of rhythmic, ritualistic transcendence via the cosmic route. (See Atriarch, Minsk and Atlas Moth among others.) Rwake takes a similar approach, making room for plenty of celestial atmospherics. A third of the way through the album’s best track, the epic 16-minute dirge, “The Culling”, everything about the band’s newfound astral vision falls into place. You’ve passed through the elegantly spiced intro with its classical guitar and swirling lysergic effects, been browbeaten by ringing riffs and scoured by vocals, until you finally reach the apex. You’re then released from its claustrophobic menace by the purifying glory of a soaring ‘80s solo.

“Was Only a Dream” ends the album on a suitably exhausting note. Spilling over with lurching crescendos and agonizing vocals, the song staggers towards the welcome pause of a spoken-word passage, in which C.T. sermonizes on nature’s majesty: “The trees are energy / Set up like ghosts so everyone one of us can breathe.” At last, the song gives way to rousing riffs and drifts away on another tweaked narrative.

While Rwake has evolved with each new release, the metamorphic leap from Voices of Omens to Rest is astounding; it’s well past easy definition now. Its inventive mix of haunting samples, momentous percussion, funeral doom lyricism, and dual guitar harmonics mark Rwake down as a leader in the arena of genuinely progressive metal. Rest is an innovative and sludgy doom feast for fans of poetic metal. Its fluctuating cadence and combination of aggression and introspection are set around a scorching core of end-times transcendentalism. Rest is solemn and intensely emotional, and it resonates with a hypnotizing cathartic energy. Rwake is anything but straightforward sludge.