though this for a moment and was shocked: NOBODY had mentioned GODFLESH

not even me

well, okay, in the end the band ended up with a live drummer and became a trio, but the most important albums (1989-1994 and again in 1999) was released and performed as duo. Hard to think of any sludge band that didn't own a lot to Godflesh.
[youtube]http://www.youtube.com/watch?v=AjFjNnfeREY[/youtube]
[youtube]http://www.youtube.com/watch?v=bL5q2qFmJEY[/youtube]
... then there's the Pittsburgh pride, Zombi, hasn't been mentioned, has it? Pretty neat, reminds me of nothing less than the Goblin

at times, which is, of course, a very, very good investment on cool.
[youtube]http://www.youtube.com/watch?v=X8VpDe-7cWM[/youtube]
I found this pretty recently, was a pleasant surprise - these chicks sure are doing the sludge the right way!
[youtube]http://www.youtube.com/watch?v=WpV3c1Laf2c[/youtube]
Oh, and: Arktau Eos, a gem amongst the ritual ambient and dark ambient acts. Instruments made of human bones, Tibetan buddhist ritual instrument and whatnot. Antti (credited as AL) used to play guitar and write the songs for the Babylon Whores. The live performances are pretty intense.
[youtube]http://www.youtube.com/watch?v=pzk8T2EyN00[/youtube]
Haeretici 7074... Rajapinta... Aghast... Abruptum was a duo for a significant part of its career... actually, I feel duo is the preferred band form in the dark ambient/ ritual ambient/ power electronics/ EBM/atmospheric black metal acts I know.
