Mercyful Fate
A Heavy Metal band from Denmark that formed in 1980, fronted by the extremely talented vocalist King Diamond. Along with Venom, Mercyful Fate's mixture of evil riffs and Satanic/occult lyrics helped inspire the Black Metal bands of the 80's, though thier music isn't usally considered "Black Metal".
Don't Break The Oath is one of the greatest metal albums of all time.