“The world is on fire,” Ben Hutcherson told an audience in Brooklyn, last month, before a set by his band, Glacial Tomb. He added, “At least we can all burn together.” Over the next thirty minutes, the band, which Hutcherson describes as playing “blackened, sludgy death metal,” roared through a half-dozen songs, replete with thundering drums and growled vocals. The set ended with a broadside of defiance, in the form of a cover of the punk band Aus-Rotten’s “Fuck Nazi Sympathy.” As Glacial Tomb sped through the song—which includes the lines “Don’t respect something that has no respect” and “Don’t give them their freedom, because they’re not going to give you yours”—audience members shouted the lyrics, churned in a mosh pit, and dove from the stage. The set was part of a fifteen-band weekend festival at Brooklyn Bazaar, in Greenpoint, called Black Flags Over Brooklyn, which was organized as probably New York City’s first anti-fascist extreme-metal show. It was planned partly as a celebration of an underground form of music that has traditionally thrived on images of drama and danger, and partly as a response to a subgenre known as National Socialist black metal, which espouses neo...