Thousand Swords is probably their most distinctive (and, I think, best) contribution to the genre. In many ways, it is an electrified Slavonic folk album composed in a narrative fashion (sort of the black metal equivalent to the debut from Darken's folk/ambient project Lord Wind). Easily one of the three or four best metal albums ever recorde. Carpathian Wolves and The Celtic Winter are also excellent, though they are a bit dependent on works by other artists (DarkThrone and Burzum) for my tastes.