Sure, I can help you think of a main riff for your black metal song. Here's a basic example that you can build on and modify to fit your own style and preferences:

e|-----------------|-----------------|-----------------|-----------------|
B|-----------------|-----------------|-----------------|-----------------|
G|-----------------|-----------------|-----------------|-----------------|
D|-----------------|-----------------|-----------------|-----------------|
A|-----------------|-----------------|-----------------|-----------------|
E|-0-0-0-0-0-0-0-0-|-5-5-5-5-5-5-5-5-|-0-0-0-0-0-0-0-0-|-5-5-5-5-5-5-5-5-|

This riff features a simple power chord progression with palm-muted downstrokes on the E and A strings, creating a heavy and aggressive sound. You can try adding different intervals or dissonant notes to create a darker or more dissonant sound. You can also experiment with different picking techniques, such as tremolo picking or alternate picking, to add speed and intensity to the riff. Finally, you can play around with dynamics, such as using staccato notes or playing the riff at different volumes, to create a sense of tension and release.
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