According to the biography, this act started out with acoustic guitars and recorded in the woods. Nature is indeed a big influence on When Bitter Spring Sleeps, most notably the mythical lives of wolves. This concept album follows his previous work, about a coven of wolves. That obviously reminds me of many neo-folk acts but this album is far from neo-folk, although I do recognize some elements.
Perhaps the music could best be described as traditional black metal with clean, power metal related vocals, driving on thrash riffs and repetitive drums. The combination of all these elements feels quite strange at first. In fact, I had some serious doubts about this album when I first listened to opener 'Witch Of November' but gradually the album started growing and gaining my interest, just like Primordial suddenly became one of my favorites.
So yes, there's the primitive black metal of bands like Marduk, the cold, heathen atmosphere of Vinterriket and the baritone vocals which remind me of Moonspell and at times even of Type O Negative. Furthermore, there's a splendid black ambient track here too, named 'Lunar Elegy'. The whole takes some getting used to but once you're into this, you start realizing that songs like 'Burning Your Eden' and title track 'Spirit In Flames' are simply great songs.