IN THE TWO DECADES and change since Metallica's landmark Master of Puppets album debuted, heavy metal has charted an ascendant course, from derided, marginalised and supposedly ridiculous youth cult to a commercial behemoth enjoying both mainstream acceptance and critical approval. But while the scene's pioneers continue to plot lucrative victory laps via the globe's enormodomes, subsequent generations have balkanised metal into myriad subgenres (Wikipedia lists 19 "primary" subgenres, leaving aside secondary sub-genres) leaving its loyal audience riven with tribal differences, unable to agree upon a true successor to the throne.