Leviathan finds Mastodon sharpening its prog-rock edge without sacrificing its grindcore muscle, resulting in a thunderous yet flexible hybrid sound. Using Moby-Dick as a recurring motif, the band summons up an aura of seafaring heroism and primeval danger (with a bit of Teutonic mythology tossed in). Brann Dailor’s tempo-shifting, jazz-influenced drumwork is often dazzling, spurring guitarists Bill Kelliher and Brent Hinds to unleash volleys of intricate, incendiary riffage. “Blood and Thunder,” “Iron Tusk,” and “I Am Ahab” couch their savagery in tight, cogent arrangements. At various points, the band slinks to a Southern boogie beat (“Aqua Dementia”), churns up a Far Eastern atmosphere (“Seabeast”), and descends into a dark, delirious swirl (“Hearts Alive”). Kelliher and Hinds spew out the album’s lyrics — heavy on battle cries and warrior laments — with a froth-mouthed desperation. The instrumental “Joseph Merrick” (named for the legendary Elephant Man) cools things down with somber acoustic guitars and distorted organ dabs.