Mercyful Fate Don’t Break the Oath was released on September 7, 1984. Back then, you most likely purchased the album on vinyl or cassette. King Diamond, the vocalist of Mercyful Fate, expanded on the image that Kiss and Alice Cooper had adopted years earlier, but he backed it up with powerful speed/heavy metal with melodic vocals and lyrics informed by more than a passing acquaintance with the occult.
If you ask me, Don’t Break the Oath was the peak of his career. The band picked up on the speed metal techniques of rhythm and used them to expand heavy metal with more fluid tempo and riff styles, then built epic songs out of that which did not over-emphasize the vocals but let them aid the songs, much like on the second Iron Maiden album. In the 1980s, just about every metal band listened to this classic.