Slipknot amp up the violence the deeper you dig into their discography. The masked marauders also diversify their attack—the ferociously percussive “Surfacing” is death-caked groove metal, and “Opium of the People” is art metal littered with breakneck changes and chattering guitars. Corey Taylor is his usual bad self throughout, invoking pure evil with a litany of screams, grunts and growls.