Recommend Me Some Underground Viking Metal

What the hell? He gets an offer before me? How fucking messed, Erik -_-

First off, Morrigan, Tyr, and Mithotyn are all good suggestions. Also add Manegarm, Glittertind, Asmegin, Iuvenes, Nachtfalke.
Yes. Or an LP, if you prefer.

Though that would mean you can no longer go "OH I OWN 0 LEGAL CD'S AND 40 TERABYTES OF MP3", are you sure you want that
Iuvenes' 'Riddle of Steel' is pretty good. It's Conan the Barbarian-esque metal. Haven't heard the other albums

Menhir isn't bad

Skyforger isn't bad either

the following bands' releases aren't heavy or fast. call it folk metal instead of viking metal

Storm (Fenriz and Satyr) - album:Nordavind.

samples there:
the first song on the 2nd album :

samples here:

Otyg (Vintersorg's folk metal band)

there are many unknown and underground bands listed in the bands section of the Viking Blood forum. most threads have links to mp3 samples