Strangest one I've ever heard was Black Doom Metal. A neighbor of mine is the composer/guitar player in such a band (Murder Rape is his band's name, in case anyone's ever heard about this style).
It is not that bad. Sounds like Black Sabbath's "Black Sabbath" song, however way deeper and with ... let's say... satanic lyrics
Besides lyrics, instrumental is pretty cool and addictive.
But, I'm still a commercial guy: Right now, listening non-stop to the album my Avatar is suggesting