If you listen to the Bathory albums chronologically you can hear Quorthon evolving from one genre he perfected to another genre he perfected. For Hammerheart he slowed his guitars and his vocals down, dropped the pitch, and then filled his music with Norse imagery. He created Viking Metal. Even now, over 20 years later I've yet to hear anything within the genre surpass this album.
Quorthon played every instrument himself. His limited vocal range was never so perfectly suited to his music than on this album. Odin listens to this as he faps.