Though no one would mistake Slipknot for optimists, the extreme darkness coursing through their fourth album may surprise even their most devoted fans. “.Execute.” preps listeners with nearly two minutes of blasting distortion, before “Gematria (The Killing Name)” crashes in with haymaker riffs that veer closer to chugging groove metal than nu-metal. As the album progresses, Slipknot get meaner and heavier. Anchoring the second half, the title track hits warp speed with piercing thrash solos and hardcore-style gang chants.