"Mercyful Fate", for better or worse, was one of the biggest influences of the Black Metal scene that openly worships Satan.

The Danish band is mostly knows at the leaping point for lead singer "King Diamond" who wore the face paint that was a demonic version of Kiss.   King Diamond is opening a Satanist, although if you look closer he's more of an atheist due to his dislike of religion.

Mercyful Fate's music is more based in Horror and madness than devil worship.  His falsetto voice is rather disturbing, and some have found it even comical.  Either way it's very unique and he continues to one of Metal's most enduring characters.

Listen to King Diamond talk about record labels and his disgust for criminals.