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Nightmare Fuel: Meshuggah
By default, most of Meshuggah's music is scary to listen to: churning guitars tuned slightly above a bass guitar, indecipherable screams, the liberal use of Mood Whiplash, and rather nightmarish lyrics, usually about death, technology, or some combination thereof. What truly is terrifying, however, is their video for "Bleed"; random cuts between a disheveled man, a dying insect, a blood-stained man chained to a wall, and something that looks like an evil albino Shiva, all over a riff that manages to sputter along in not quite 4/4, which just adds to the unnerving "I should be able to see a pattern here" part of the brain.
Another potent example is "Mind's Mirrors" off of Catch Thirtythree. Listen to that in a dark room alone, and try not to get freaked out, especially with the extra low, distorted bass in the beginning. Brrr.....
"Elastic"; the song starts out like any other Meshuggah song, but ends in a breakdown/solo thing with a dreadful-sounding guitar, then devolves into about 6 minutes of drone ambiance, then proceeds to play every previous song off of the album Chaosphere (including the aforementioned New Millennium Cyanide Christ)...at once. This extremely sudden Mood Whiplash (not to mention the whole song itself) can be very disorienting and unbearable
The heaviest, most intense and overwhelming set of riffs the band has ever written appears out of nowhere after a half-second break in "Demiurge". It is extremely jarring for those unfamiliar with the song's structure.