Formed by Stephen O'Malley and Greg Anderson in 1998, this is possibly the heaviest band you will ever listen to.
Noted for their extremely loud shows filled with so much fog you can barely see, the music seems to resonate more from within you than from the speakers in front of you. This guy has a very good video about his experience actually, and it's worth a watch.
Playing in the excessively low tuning of Drop A on average and with Attila Csihar of Mayhem as their primary vocalist since 2003, I would say there's possibly no one who is quite the same.
Though not exactly "evil", they are certainly dark and foreboding. This 25 minute song for example, deals with a great many things. Though it should be noted that Attila is not the vocalist on this track.
Honestly, there's not too much to say about these guys. Other than being the loudest thing around and wearing robes all the time of course. I personally can't wait to see them someday.
This album was also named the 18th heaviest album ever by Q Magazine.