Niflheim is the realm of the unfortunate dead in Norse mythology. Its counterpart is Valhalla, where only the lucky dead go. Niflheim and Valhalla have other things in common; both are vastly wide lands, populated mostly by the dead, and each has a great hall. But there are differences:

Niflheim is ruled by Hel, who never leaves it, at least not that the eddas mention (although the eddas do not say that she is trapped there, and other rulers may leave their lands).
Valhalla is ruled by Odin, although he is not known to have any direct involvement in it. But the Valkyrie and the Einherjar, who reside in Valhalla, are all sworn to Odin.

Niflheim has a hall called Eljudner.
Valhalla has a hall called Elvidner.

Niflheim is eternally dark.
Valhalla enjoys a sunrise every day.

The name Niflheim, also pronounced Niflheimr, means 'Mist Home' (or 'Abode of Mist' or 'Mist World'), and it is one of the Nine Worlds in Norse mythology. Along with Muspell, Niflheim existed before the gods, and it was from the meeting of Muspell, the land of fire, and Niflheim, the land of ice, that the first rivers were spawned from a universe of fire and ice ... and on the banks of these rivers, life began.

I have always believed that the concept of worlds melting out of ice arose from survival in the harsh climates of early Scandinavia, which were suddenly given the first recorded decent summers (for farming) during the Great Migration Age of 400 to 800 AD, when the population of Scandinavia grew from scattered tribes, barely eking out an existence in a frozen land, to large populations of strong nations. Then, just as Scandinavia arose to be considered a populated land, it was hit harder than most lands by the extreme weather events of 535–536 AD, which were the most severe and protracted short-term episodes of cooling in the Northern Hemisphere in the last 2,000 years, causing unseasonal weather, crop failures, and worldwide famines.

Recovering from this sudden climactic harshness, numerous diverse Norse cultures explosively flourished, only to find their glacier-capped lands suddenly too small to support their large populations, and this led to the Viking Age.

Note: The Viking Age ended before the Little Ice Age (1550 to 1850 AD), so claims that weather was partially responsible for the end of the Viking Age are completely false.

Back to Niflheim:
The main purpose of Niflheim is to contain all of those poor souls who died by any means other than fighting. Most texts refer to a 'glorious death in battle' as the only means by which any mortal can escape eternal doom in Niflheim. This shines a light on Old Norse thought: being a warrior was praised above all other occupations. This supposedly spelled doom for all farmers, merchants, craftsmen, horse trainers, cooks, and especially women; the Norse outlook of these people, if they truly believed in their religion, must have been crushingly bleak.

Strangely, this high-status of the warrior may have been a religious truism in the Norse eddas, but it seems to be missing in the Norse Sagas. Many notable men in the Norse Sagas are not warriors; some are scholars or wise kings or men of great wealth (some were proud to be farmers!). The Norse Sagas were oral traditions which loudly praised warriors and warrior-kings, but there are many instances of powerful men who didn't seem driven by the fear of not dying in battle.

Yet, I didn't write The VIKINGS! Trilogy to be a research paper; multiple interpretations of historical precedents would have muddled the story. I was writing about Norse mythology and chose the description of Niflheim that was the oldest and, to me, the most fun; I make no apology for this.

Many of my details of Niflheim really are from Norse mythology: Garm, Nidhogg, the River Gjoll, Eljudnir, Ganglot and Ganglotti, even Glimmering Misfortune (the curtains around Hel's bed) are described in both eddas.

One thing that I didn't keep was the mists; thick mists would have hidden the vastness of Niflheim, which I wanted the companions to see. (I chose to keep out the mists for a very important reason which I will not reveal here.) Also, 'Niflhel' is one mysterious place in Niflheim that I purposefully did not describe in any detail; almost nothing is written about it in the eddas, and I am leaving that for future story ideas.

The worst part of Niflheim is that everyone who ends up there is doomed to side with Loki in Ragnarrock, to rise in the final battle as part of the forces of evil, and wage war against the forces of good. This means that anyone who is not a warrior is destined to be evil ... and thus all warriors are good. As a knight in the SCA, I like this part best ... even if some claim that it isn't true.