Bmiler
I wouldn't call this metal, but the occult nature of this album is probably why this might be thought of as "black metal" but in reality, it's about as metal as Jethro Tull. In fact the music isn't too far off from Tull territory, especially from the flute playing. But unlike Tull, the group also used a saxophone (Clive Jones is responsible for all wind instruments here). The music is basically early '70s organ/sax/flute driven prog rock with Satanic themes in the songs. I find it an enjoyable album, but in mixed company I would not dare play this. "In Ancient Ways" is a nice opening piece, with acoustic guitars and sax. I really like the organ work from Zoot Taylor. "Come to the Sabbat" features some cool drum beats, plus that chant that goes, "Come, come, come to the Sabbath, come to the Sabbat, Satan's there". There's some orchestration that reminds me of David Palmer's orchestrations in Jethro Tull. I like that bolero beat in "Conjuration", while "Attack of the Demon" is another favorite of mine, about a sorcerer who is doomed to hell. The title track is the lengthiest piece on the album which includes some nice jazzy passages.
The content and their stage shows were a source of controversy, including a mock sacrifice. Many Christian priests tried to woo kids away from attending their concerts. I am a proud owner of Demons of the Night Gather to See Black Widow Live, which includes a bonus DVD from a 1970 German TV broadcast (I am really surprised German TV would air something as controversial as that in 1970), so I get to see what all the controversy is about. This controversy caused the band to drop their occult image, so none of their following albums have any Satanic themes.
So warning: it's not heavy, don't expect something on the lines of Black Sabbath, but if you like early British prog, you'll probably have no problem with it.