It's common knowledge that black metal swept in, full force, in the mid 90's just when Death Metal was getting a little old in the public eye. The 'search for extremity' certainly came to a momentary halt when bands like Blasphemy, Burzum, Dartkhrone, Gorgoroth, and Emperor came crawling upwards from the lower echelons of obscure and extreme music. These bands weren't only raw in instrumental execution - the image and general content was as well. Bands were sometimes emotionally raw, expressing pure human dread, misery, and insanity through the sound itself. When black metal was angry or disgusted, it aimed to shred the fabric of the offense itself. And, when mentioned that otherworldly powers, whatever they may be, were key to the beliefs and music of these bands - it rang shockingly true (even if it wasn't always the truth). You'd also hear about events where someone did something shocking or extreme (like killing their friend) and there was the overwhelming possibility that it could be true.
A lot could say the general pageantry and uniform of black metal played into it's extremity and appeal, but not all the bands had it - which is still the case even now. Even Emperor has surprisingly few instances of makeup and corpse paint compared to even older bands that weren't even black metal to begin with. Most of it was black and white photos, or robes, and a few instances of armor showing up here and there during live shows. Though, black metal shows weren't incredibly common back in the day - nor are they even now, when the vast majority of people are at least aware the subgenre exists.
Anyhow, while you got your super low-fi bands that made due with their message and limited playing skills - you had things like Emperor and their first album 'In The Nightside Eclipse.' And, for anyone who'd been listening to any other black metal band at the time - this album had to have blown their expectations for the subgenre out of the water. Even now, In The Nightshade Eclipse is leaps and bounds ahead of most releases. There was a lot of thought and care put into this album, a almost obscene amount. Between all the noise and distortion, there was somehow room for structure and grace. Where a song could be speed-heavy and fast paced, Emperor could slow it all down and still make it sound sinister. All that synth and layering - what was this doing in black metal? Why hadn't it always been here?
Of course, Emperor went on to do bigger and more experimental things. Eventually, the bands last album would scarcely be considered Black Metal. Ihsahn, of course, went on to blow peoples minds with extreme progressive metal that would span many genres and only get better as time went on.