Originally Posted by wmheilma
Diatonic harmony same as everyone else with a bit of extra time on Phrygian and Aolian modes. Then move on to arpeggios of the harmonic minor scale. Minor pentatonic also is used a lot. Phrygian Dominant mode is another good one to practice.
This. In my experience, and I used to play a lot of metal in my beginning stages, you don't need to go any further than minor pentatonics. In most bands you'll be riding the root anyway or if you play anything with blues influence like stoner, noodle some minor pentatonic underneath.
It's another bag of tricks if you want to compose. There is a lot of fun to be had with modes and key changes, chord substitutions, outside playing and so forth. And that's where you leave classical heavy metal territory. You are now in the Opeth realms.
But I digress. Essential knowledge to me would be chord construction (7th chords), modes, the three variants of minor (natural, harmonic, melodic), minor pentatonic. These will also come in handy should you ever take a solo (and chances are you won't)