Satanic/pagan imagery has always been a staple of Heavy Metal, but when it comes to Black Metal (wikipedia, allmusic) these guys love them some Satan.  Actually, that's a bit of an oversimplification...some of these bands might indeed be true Satan-worshippers, but many are more anti-church than they are pro-Satan (particularly the Norwegian bands).  In addition to being ardent Devil-worshippers and/or church haters, Black Metal artists take shock value to the extreme, to the extent that some have been involved in church burnings and murder.  Stylistically, Black Metal has strong connections to Death Metal, although in many ways Black Metal is a response by bands who didn't think Death Metal was extreme enough (!).  To be perfectly honest, I'm not that fond of many of the tracks I've heard from this subgenre, although I think the musicians involved would be happy about that, since this music is specifically designed to be off-putting.  Nonetheless, there are some tracks that I do enjoy (although I don't necessarily endorse their worldview).