Heaviness is a fickle descriptive when it comes to music. Is it gauged by the power riffs of a Black Sabbath or Led Zeppelin? The hyper-speed assault of a Slayer or Napalm Death? Maybe the slow, brutal chug of a Swans or Neurosis? Some even look to the dark undercurrent of early bluesmen like Blind Willie Johnson or Leadbelly (the name certainly checks out) as the true masters of heaviness.

Sumac frontman Aaron Turner has described his band as an attempt to create the heaviest music ever written. Even if no one can really agree on what the hell heaviness is, Turner certainly does have the pedigree to pull off his claim. 

With his seminal band Isis, the singer-guitarist helped establish post-metal, which combined post-rock, heavy metal and shoegaze into something more crushing than the sum of its parts. After disbanding Isis a few years back, Turner formed Sumac, recruiting one helluva cast to support him in his quest for heavy. Bassist Brian Cook, with the decade he’s logged in Russian Circles, brings a proggy experimentalism and rhythmic coherence to the band’s low end. 

Filling out the rhythm section is drummer Nick Yacyshyn, whose drum-kit assaults with Vancouver crust-punk outfit Baptists earned him raves from the likes of Dave Grohl. The trio’s first show together came in December 2014, opening for metal band Deafheaven — an auspicious start, to be sure. 

Together, those post-metal, prog and crust punk influences have coalesced into something lumbering, brutal and, yes, heavy. Sumac’s debut from last year, The Deal, saw the band stretching tracks — “Thorn In Lion’s Paw” and “Hollow King” were glacial in size, showcasing Turner’s ear for primal riffs and the band’s gift for gradually ratcheting up tension. 

There’s also a wandering, improvisational feel to tracks like “Blight’s End Angel,” which suggest the band will take a similarly exploratory approach when it aurally assaults the tiny Boreal venue May 23.

The Louisville, Kentucky-based Jaye Jayle will open with his drunken blues and psychedelic Americana stylings. It should make for an intriguing counterpoint — but then again, heaviness comes in all forms. 

Sumac and Jaye Jayle perform 7 to 10 pm Monday, May 23, at The Boreal; $8, all ages. — J.D. Swerzenski