With the one-two gut punch of Brent Hinds's departure from the band, then his sudden, tragic passing less than six months later, Mastodon's first record without his input was always going to be a statement of bereavement. However, it's unexpectedly also their heaviest record since 2004's Leviathan. Spurred on by the loss of the incredibly talented but incendiary guitarist-vocalist, the band has crafted the darkest set of songs in their catalogue. Even though they've added longtime touring keyboardist João Nogueira and Canadian prog guitarist Nick Johnston to their songwriting and recording process, the seminal sludge metal band have also chosen to bring on a slew of absolutely wild collaborators across Marrow Deep. This ninth outing by the singular guitar gods of '00s metal might be heavy music's answer to Phoebe Bridgers's massive crowd of folk features on Lost Weekend. While the record may lose some urgency toward the last few songs, its opener couldn't be more immediate: "Barbarians Blood" chugs and twirls across the fretboard interchangeably, buoyed by the tune's featured drummer, Converge's Ben Koller, insisting on a breakneck pace for this celebration of "a really good ole' fashioned 0-0-0 riff." Now that main drummer Brann Dailor and bassist Troy Sanders are carrying lead vocal duties without Hinds, it's Sanders who has chosen to fill in the parts that Hinds would've normally snarled his way through. Sanders has really never sounded better, and his gritty yowls and powerful shouts balance out Dailor's clean, wailing vibrato on tracks like the melodic, vocally elastic "Poisonous Weapons." It may disappoint some that Queens of the Stone Age's Josh Homme isn't more audible on "Snakes for Dinner," singing the final coda but depriving us of the trademark scale and tone of his guitar playing. Considering the last time Homme featured on a Mastodon song (the modern metal classic "Colony of Birchmen"), it's a shame he isn't put to a little more use here. That said, the chorus of "Snakes" is one of the most directly upsetting moments on the record: "When you left you took our hearts and minds / All that I could find amongst the grime were memories," croons Dailor. The same angry feelings of regret permeate a deeper cut, the hardcore-tinged "A Vampire's Demeanour," which features vocalist Randy Blythe (Lamb of God) and bassist Nate Newton (Converge). Sanders explodes with Hinds-like ferocity: "I saw you crumble like an avalanche / Screaming at the sky!" The most fascinating change-up on the record is the band's employment of some absolutely savage black metal sounds on songs like "Out Like a Lamb" and "Golden Spires." The main riffs of these tunes are tremolo-picked with an icy ferocity that would put '90s church burners to shame. These tracks dwell in a motorik but doom-laden atmosphere, with the sound of the band's mourning producing some of Mastodon's most original songwriting in years. The gentle, flighty reprieve of "Moth & Bone" is striking, if a little too digital-sounding in the synth work. "The Vanishing" is a blues rock tribute to Hinds, with perhaps the two single wildest features ever put on a rock song: Black Sabbath's Geezer Butler starts the tune off with his trademark fuzzed out groove, while Gojira's Joe Duplantier arrives at the climax, laying down his guttural, lung shredding screams. It's bone-chilling, and the entire song serves as the emotional payoff to a record that meditates on loss; it's only fitting that metal legends from the earliest and most recent periods of the genre's history share the stage here. If there's one thing that's missed most of all on Marrow Deep, it's Hinds's one-of-a-kind hybrid fusion of country and metal. Gone are the bizarre, half-major, half-minor pull-off runs, the chicken picking and the swampy solos — and unfortunately, they've largely been replaced with a sort of interchangeable spacey "prog" noodling, and that sort of playing can be found in any number of lesser rate bands. Take the final track, the lengthy "The Three Fates," where increasingly jazzy keys and a never-ending solo simply bleed the momentum dry. Anytime the band slow down, there's a good chance of something going awry, like on the mid-tempo space rock of "They're Coming for You." These songs' subject matter doesn't even justify their muddy, meandering pace, as some of the most fiery moments on the record feel like the most fitting tribute to Hinds's relentless nature. For better and worse, the tragic circumstances of the album's creation are infused in its veins. There's a sonic void where Brent Hinds once shredded and wailed his way into our hearts, but his passing has also inspired some of the most emotionally hefty material Mastodon have released in over a decade. Adding two musicians and a slew of collaborators has opened the band up to some influences they've sorely missed: hardcore, black metal and '60s proto-metal. The songwriting is more winding than ever, and for fans of sonic whiplash, it's here in spades. Marrow Deep is largely a triumph, a fitting tribute to their fallen founding father — and to heavy metal itself.