Don't Break the Oath
In the 1980s, there were a lot of bands in Europe playing heavy metal.  Many of them were playing music like bands from the United Kingdom, in a way that was called the New Wave of British Heavy Metal (or NWOBHM for short).  There were a lot of hard rock bands, too, and there were a lot of things hard rock bands and NWOBHM bands both did.  Mercyful Fate was one of the first few NWOBHM bands to be different from hard rock bands on purpose.