cracked skulls - this is good for almost any kind of band. maybe it's a little weird for a folk duo, but it's still totally works.
if you're want something a bit more aggressive for your hardcore or death metal band --
and if, like me, you are an unrepentant fan of cephalopods --
unfortunately, if you're like me and kind of a nerd, it might bother you to think of kraken skulls, since likely krakens don't have skulls, just fearsomely-strong beak-like jaws. but i'd let it go if your band was awesome.