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Limited picture disc vinyl LP pressing. Don't Break the Oath is the second studio album by Danish heavy metal band Mercyful Fate, released in 1984. The style Mercyful Fate employed on Don't Break the Oath resembled a mixture of heavy metal with progressive elements, lyrically preoccupied with Satan and the occult and distinguished by King Diamond's theatrical falsetto vocals. Although very influential to future black metal bands due to it's lyrical content, the music itself is more reminiscent of traditional heavy metal. Metal-Rules. Com named this the greatest extreme metal album of all time.