Plenty of death/doom metal aesthetics and the like are influenced by high/dark fantasy, which a Tiefling makes especially easy to pull off. Just browse album art until you find one you like and then say "that's me!".
But instead, I imagine you're basically gonna play Eddie Riggs from Brutal Legend. Granted, that's more your garden-variety heavy metal look, but look at the demon or goth faction art for ideas. Bonus points for Valor Bard stringing up a literal axe as a guitar - extra bonus points for convincing your DM to let you do thunder damage and making your battle cry "THESE GO TO 11".