In the 90s it became clear that soft/hard was a winning formula. Bands like Nirvana later capitalized on how to write a gentle acoustic verse and then have a wailing chorus. But before that, metal bands experimented with this, possibly starting with Venom's "Cold Northern Breeze" but maybe dating back to some of the Black Sabbath experiments.
Cemetary took this into death metal, mixing a ghostly fast vibrato (I still don't know how they did this technique) with lighter distortion than most, creating a sound like veils draped over the howl of wind through a cave. Then, they added in acoustic guitar used strategically at points of quiet emotion, then built up to the more raging death metal riffs. Far subtler than the soft-verse/hard-chorus that Nirvana, nu-metal, etc. would use, this technique caught the attention of many for its subtlety and emotion.
Then Opeth cloned it in a simplistic form, following the nu-metal option.