Black Metal its purist form. Used to reference bands such as Emperor, Mayhem, DarkThrone, Burzum, and more recently Gorgoroth and Immortal. Newer bands in the third wave of black metal do NOT qualify as True Norwegian Black Metal. (sorry Dimmu)
Burzum represents the end of the original wave of Norwegian Black Metal. Varg Vikernes style consisted of far too much ambience to be considered by most to be "true Norwegian Black Metal", but still to his fans he is the most true musician to date. For better or worse, its in the past. Living on today are still many great bands in the genre, and there isn't an end in sight.