Mastodon's First Album Since Brent Hinds' Death Features a Surprise Geezer Butler Bass Cameo

Geezer Butler sent Mastodon a bass solo, and Troy Sanders is not shy about what it means for the record. "There's definitely a bass highlight on this record," Sanders told Bass Player. "But it isn't me." That solo opens "The Vanishing," track 11 on Marrow Deep, Mastodon's ninth studio album and the first the band has made since founding guitarist Brent Hinds' death in August 2025.

Butler, Black Sabbath's bassist and co-founder from 1968 to 2017, recorded his part cold: Mastodon sent him a batch of songs with no specific brief, and he sent back a solo Sanders now calls the album's defining low-end moment. Nobody outside the band has heard the finished mix. Marrow Deep arrives August 28, 2026 on Loma Vista Recordings.

The invitation traces back to Black Sabbath's 2025 farewell show. "We'd just played the Back to the Beginning show and I thought, 'What's the worst that can happen if we ask Geezer to guest on our album? He could say no?'" Sanders said. The band already had a reason to think Butler might say yes: he'd namechecked Mastodon as a band he enjoyed listening to in his own autobiography, a detail Sanders said "blew our minds."

What followed was entirely arm's-length. "Everything went through management. We've never actually met face to face," Sanders said. Even at Black Sabbath's Villa Park farewell show, the two camps never connected in person, "it was a madhouse in the Sabbath area, so the idea of trying to go back there just didn't feel right." Sanders is hoping to finally meet Butler on tour later this year, when Mastodon's own dates land near one of his.

The Butler cameo is one piece of a bigger relaunch. Marrow Deep is Mastodon's first release as a five-piece: drummer Brann Dailor, bassist Troy Sanders, and guitarist Bill Kelliher are joined by new members Nick Johnston on guitar and João Nogueira (of The Claypool Lennon Delirium) on keyboards, per Louder. The album was co-produced at West End Sound in Atlanta with Patrik Berger and Converge guitarist Kurt Ballou.

Homme's turn on "Snakes for Dinner" is a callback, not a first. He guested on Mastodon's Blood Mountain in 2006, singing on "Colony of Birchmen," twenty years before this second appearance.

Founding guitarist Brent Hinds parted ways with Mastodon in March 2025, in what the band described as a mutual decision; Hinds later said publicly he'd been "kicked out." He died on August 20, 2025, at age 51, after a collision with an SUV while riding his motorcycle in Atlanta, per NPR and Billboard.

Marrow Deep's first single, "Your Ghost Again," was dedicated to Hinds. Sanders described the sessions as carrying some of the energy of Mastodon's earliest days: "It's reminiscent of the very beginning of our band, where all members are hungry, we're united, and excited to get to work," he said, crediting the new lineup for the feeling.

A guest cameo doesn't come with a fresh set of documented gear, but Butler's own rig already has a long paper trail, and CYS has it. Geezer Butler's bassist profile documents the two eras: Rotosound Swing Bass 66 (.045 to .105, bright stainless roundwound) across Black Sabbath's 1970s commercial peak, and DR Strings (the Black Beauties set) as his current endorsement. That downtuned, heavily-distorted low end, doubling Tony Iommi's guitar at bass frequencies, is the same tone that gave doom and sludge metal its low-end blueprint, the genre lane Mastodon themselves work in.

Worth being precise about what this cameo is and isn't: it's a one-song guest credit on an album nobody has heard yet, not a Butler-Mastodon collaboration record, and not a lineup change. Whatever tuning and gauge choices Sanders and the rest of Mastodon settled on for Marrow Deep haven't been published anywhere CYS could verify, so we're not guessing at them here. If you're chasing that downtuned bass tone yourself, rather than Mastodon's specific setup, CYS's guide to gauging bass strings for Drop D, Drop C, and lower and best bass guitar strings comparison cover the gear side in depth.