What Borknagar should I get next, I already have Olden Domain.
You have a choice of going backwards and getting the self-titled debut, which is simpler and more black-metally that
The Olden Domain but has the same vocalist, Garm, who shrieks most of the time but does some chants here and there. The album has a wonderful atmospheric production.
Or go forward with
The Archaic Course, an album that takes
The Olden Domain's sound further, and in my opinion improves upon it, and it introduces a new vocalist, Vortex (best clean vox in extreme metal imo), who sings about 2/3 of the time and growls the other third.
After you fall in love with either or both of these albums, naturally you should go forward and pick up
Quintessence, which keeps the same vocalist but the guitar sound and musical direction changes much. Certainly Borknagar's most intense album, it represents the band's creative peak before the more progressive direction carries the later albums
Empiricism and
Epic, which have another new vocalist, Vintersorg.
And if you love acoustics with your folk metal, be sure to check out
Origin, the band's latest album.