they are something of a prog-doom-metal-of-the-90's answer to 60's/70-s Pink Floyd and King Crimson.
started off as a atmospheric pagan black metal (in the vein of Ulver's Bergtatt or early Bathory) on Heart Of The Ages (1995), but evolved into much greater thing.
try to find Omnio (1997) and Strange In Stereo (1999) albums, really unique stuff.
i remember Mikael himself saying in some interview back in 1999 he pretty much liked Strange In Stereo too