Mixing black metal with folk music doesn't make them unique, but their sound has a more...well, folky quality to it than the epic Viking type stuff. I generally dislike black metal (apart from some of the album covers) but in Finntroll's case, the folk elements are strong enough to make them sufficiently different. Their lineup has suffered the loss of two vocalists; one passed away and one was expelled from the band not long ago.
I've been listening to a lot of KAMPFAR lately. Excellent black viking metal. Fra Underverdenen is an instant classic. Every song oo this album is a gem. Mollen Skogledde Aeser is also very good. Kvass isn't as amazing as these other two but is still good. MUST HEAR!!
It's a PANDEMIC! It's a metal disease... PANDEMIC! Brain cell abduction, creeping in like sin, it's a PANDEMIC! 'A metal disease... PANDEMIC! Brain cell destruction, breeding metal from within... Whaaaa!!!!