Before there was "kvlt" there was Mayhem. Their influence would go on to become transmusical, but in the beginning that's all there was: a six song, 18-minute EP whose genius was equal parts cunning, refusal to compromise, and a handful of Norwegians who didn't have the slightest fucking clue what they were doing.
Deathcrush was self-released through founder Øystein "Euronymous" Aarseth's Posercorpse Records in August 1987, "marketed" – to use the term loosely – strictly toward friends and a small sect of like minded death metal addicts in the local area. Yes, Mayhem were very much a mortality-obsessed outfit in those early days, taking cues from the likes of Venom, Hellhammer and Bathory and seeking to one up them with ultra-gory lyrics ("Chainsaw Gutsfuck") and onstage shock tactics, including one of the earliest uses of "corpse" paint… as differentiated from the more Kabuki-like schtick of Kiss and King Diamond before them.
In that regard, it was bandmate Per Yngve Ohlin, more preferably known as Dead, who was probably most responsible for the sheer extremity of Mayhem's early energy. It was Dead that standardized the corpsepaint look for later bands that would both ape and seek to expand upon Mayhem's aesthetic, and to add an authenticity to the postmortem look he went so far as to bury his clothes for several days before a concert to give them a moldering, fresh-from-the-grave appearance.
Dead was also one of the first – if not the first – in the extreme metal scene to resort to cutting himself on stage for effect, and the band's use of severed pigs heads and various dead animal parts also set the bar for what would later coalesce into the second wave of black metal.
The Deathcrush EP is at once groundbreaking and regressive; while certainly establishing new standards of brutality, the songwriting itself is still largely derivative of their influences, particularly Venom and early Bathory. The EP actually includes a sped up cover of Venom's "Witching Hour", and the title track itself ping pongs back and forth between a midtempo, heavily NWOBHM-inspired riff, and the blastbeat driven, sloppy thrash that early black metal would become known for. Closing track "Pure Fucking Armageddon" follows the latter formula, although upping the ante substantially in terms of aggression.
The real trump card here is the alternating vocals of Messiah and Maniac, each of whom individually represent unprecedented levels of brutality and unhinged hatred than anything that had been seen up until that time (the closest by far being Quorthon of Bathory). It's probably overstating it a bit to say that, without these two vocalists, Deathcrush would have been blown off by history as an egregiously sloppy thrash demo, but their vocal contributions undoubtedly propelled the rest of the band – and, afterward, the Norwegian metal scene as a whole – into further heights of depravity.
From this point on it would become an almost pissing contest to see who could be the most brutal, "kvlt" black metal act out there, but it's important to realize that back then terms like "black metal" and "death metal" were general descriptors at best; they weren't in widespread use as acknowledged genres of their own. Even after Deathcrush it would take several more years of scene cultivation before anything resembling an actual movement would amalgamate, but this right here is the arch-root of it all.