Because almost all their songs are either about Norse mythology or Viking culture, the Swedish death metal band Amon Amarth often get mislabeled as Viking metal. Vocal about refuting this, frontman Johan Hegg once said, "I can't imagine the Vikings were into metal at all, except on the swords and stuff."
Pretty funny. The truth is, Viking metal is a very small, super-weird and utterly glorious subgenre of extreme metal that was basically invented by Norwegian teen Quorthon and his band, Bathory, with the release of their 1988 album, Blood Fire Death. Bathory's previous three albums had laid the blueprints for black metal, but on this one, Quorthon pushed out the borders of the music, allowing for expansive, epic songs by slowing tempos, mixing in Scandinavian folk-guitar parts, adding the sound of horses galloping, and spinning lyrics about death on the battlefield, destroying villages, food cooked on an open flame, etc. Then came 1990's Hammerheart: Quorthon dropped the wraith-screech vocal style of his past, simplified the riffs from a black metal pick-flutter to a chunky heaviness and added some chanting hordes, whereupon what we know as Viking metal was truly born.
Formed in 1991 (and also teenaged and Norwegian), Enslaved's own trio of Viking metal albums -- Frost, Eld and Blodhemn -- expanded the definition of this tiny genre with prog rock elements, pre-battle speeches presumably recorded in dank throne rooms and most importantly, as it was a staple of true Viking music, the mouth harp. They also tossed in alternatingly cheesy and clubby synths. These guys got up to some weirdness, but their music is flat-out awesome: just check out their album covers, or listen to all of Frost and "793 (Slaget om Lindisfarne)" from Eld. Unfortunately, from there bands marked as Viking metal tend to drop off in quality. One thing that makes the genre great is its lack of subtlety, but that doesn't mean it isn't possible to go a little over the top with the chanted vocals and mouth harps. What follows is a list of the Bathory and Enslaved records that both define Viking metal and represent the best it has to offer, which is saying a lot, because this music sounds great when you crank it and goes almost too well with Skyrim. There is also an album at the end that kind of identifies the limits of the genre. If you want to go deeper, you might want to check out Mortiis' Født til å Herske, but you may have to actually be a wood elf to get it. Okay, have fun.