Once upon a time there was hard rock - rock played with a bit more oomph, with a bit more swagger, some faster tempos and some histrionic screaming. Led Zeppelin turned Jimmy Page's two-necked guitar, Bonzo's thudding twenty six inch Ludwig Acrylic double bass drums and Robert Plant's amazing vocal range into a monster best seller, creating a larger than life lifestyle involving personal Lear jets and sticking fish into the orifices of underage groupies.
Then Black Sabbath had a bit of tragedy: on the eve of them going from budding musicians with day jobs to actual signed musicians - in fact, guitarist Tony Iommi's last day on the job, he managed to sever the tips of the fingers of his right hand in a metal brake, requiring him not only to quickly relearn the guitar on his non-dominant side, but learn to play with prosthetic fingertips. To make the job slightly easier, he de-stressed his finger requirements by drop tuning his guitars, making them play at lower pitches and giving their droning, laid back sound a "doomy" character. It seems silly to think that a pitch change can matter, but Robert Smith of formerly mope-merchants "The Cure" tunes his guitars slightly higher and plays slightly faster than the recorded version so when pitched down it has a "sparklier" sound to go with his newer, more upbeat music.
It continued in this vein for a while before taking some slight detours: being maintstream music in the 1980s, being mixed in with rap, and so forth. Some folks decided to play darker, starting with Venom out of Newcastle, England. Their shtick was Satan worshipping and turning a trio of frankly bad musicians into the progenitors of an entire "eviler than thou" Black Metal scene. The Norwegians loved the idea of sounding terrible and advanced the art of sounding awful by doing things like recording tracks with cheap headphone speakers repurposed as microphones to eliminate the possibility of even sound quality. Thrash metal took the idea of unfocussed playing, in opposition to speed metal which tried to be clean, crisp and fast. Thrash guys found out they had a lot in common with second wave punk rock, before punk became something 14 year olds bought at Hot Topic and developed crossover metal.
Then the Scandinavians and Europeans decided that Satanism was basically a silly inverse Christianity and started things like folk metal and tree metal in which they crossbred metal with "Scarborough fair" and a lot of Dungeons and Dragons imagery.
The bedrooms of many a guitar nerd were cooking up new ways to push the envelope in ways it was already going.
An acoustic guitar is almost pointless to play with something muffling the strings. An electric guitar is another matter. A guitar played with the fleshy back part of the hand pressed into the strings mutes them from ringing while the thumb and forefinger pick furiously produces a characteristic "chugging" sound Metallica brought to the masses. Pressed even harder and played very staccato produces an almost percussive sound.
Someone decided it sounded sort of like someone saying "Djent".
In the quest to push the envelope, you can only go so far in a given direction. There's only so fast you can play before it all sounds like a jackhammer rather than music. There's a limit to how much virtuosity someone can string notes together with before it sounds like the drone of a bumblebee's wing. There's only so far you can push lyrics and song titles until they sound like parodies of themselves. "Hammer Smashed Face" works as a song title. "Impaled Goat Vaginal Putrid Rotting Cervix Evisceration" is trying too hard.
So to push the envelope, the internet nerds went in three directions at once.
The first was to make their playing very percussive in the rhythm guitar parts. Hence, "Djent". With advanced computer based effects, you can get a distorted (in the sense of overdriven sound) tone that's very clean and ringing. That was step one.
The second was to play in off meters, play with an attention to technical excellence, and an attention to different scale ideas and such. That was step two.
The third was to decide that six strings were not enough. You can only detune a guitar so far before it starts to become unplayable. Therefore they sought out custom guitars with seven, eight, or even nine strings whose string thickness started to approach that of the bass guitar. This allows for a lot of low, growly aggressive sounds and ominous sounding low register atonal sounding melody lines that would be muddied by normal drop tuning.
One of the first bands to successfully take it out of the realm of "subscribe to my Youtube Channel" and onto actual records was Meshuggah, whose most famous track "Bleed" off "ObZen" showcased a song with a driving, "duggada da duggada da" one note line in the bass register that drones up in a four-semitone blend before falling back to the original note. It made the characteristic drone riff from "Zero the Hero" by Black Sabbath appear almost cheerful by comparison. Anchoring all this is a drummer playing an odd meter on double bass drums reminiscent of part of the "school bus engine" riff by Alex Van Halen from the intro to "Hot for Teacher" while he crash rides away like a demented Keith Moon.
Speed couldn't do it. Volume couldn't do it. Unfocussed flailing couldn't do it. But they managed to find a way to make heavy metal even harder.
It's a niche metal genre and I'm not going to bother to even try to find out who else is playing this. But it deserves a mention and some kudos for thinking outside the box and giving the dying carcass called rock and roll a new palette of sounds in his dying days.