Dream big, aim high, and unleash the beast within. That’s the philosophy behind Mastodon’s mutant form of prog-metallurgy, as evidenced on 2006’s Blood Mountain. Singer/guitarist Brent Hinds rages like a maddened prophet confronting every Angel of Hell imaginable, howling himself raw with defiance. “Crystal Skull,” “Colony of Birchmen,” “Hand of Stone” and similar tracks invoke gore-spattered Nordic tales of the supernatural, brought to seething life with shifting tempos and stabbing guitar strokes. Beneath the album’s volcanic surface, there is surprising nuance, revealed in the jazzy undercurrents of “Capillarian Crest” and the hyperkinetic grandeur of “Bladecatcher.” “Sleeping Giant” stretches forth like a majestic arctic sea, while “Circle of Cysquatch” throbs with the fury of a flagellation ritual. Mastodon fully struts its elephantine stuff on “Pendulous Skin,” a vast, multi-textured sojourn into the band’s inner universe. At times, Blood Mountain nearly sinks into aural murkiness and lyric obscurity. Admittedly, the album is a lot to digest — but for those seeking epochal metal with a dark twist, this sprawling work offers massive satisfaction.