Niflheimr or Niflheim ("Mist Home", the "Abode of Mist" or "Mist World") ; Nifl being cognate with the Old English Nifol ("dark") and Nebel, a German and Latin root meaning cloud) is a location in Norse mythology which overlaps with the notions of Niflhel and Hel. The name Niflheimr only appears in two extant sources and they are Gylfaginning and the much debated Hrafnagaldr Óðins.
According to Gylfaginning, it was one of the two primordial realms, the other one being Muspelheim, the realm of fire. Between these two realms of cold and heat, creation began. Later, it became the location of Hel, the abode of those who did not die a heroic death.
It was many ages before the earth was shaped that the Mist-World [Niflheimr] was made; and midmost within it lies the well that is called Hvergelmir, from which spring the rivers called Svöl, Gunnthrá, Fjörm, Fimbulthul, Slídr and Hríd, Sylgr and Ylgr, Víd, Leiptr; Gjöll is hard by Hel-gates.Odin (as Þriði) further tells Gylfi that it was when the ice from Niflheimr met the flames from Muspelheimr that creation began and Ymir was formed:
Just as cold arose out of Nifiheim [Niflheim], and all terrible things, so also all that looked toward Múspellheim became hot and glowing; but Ginnungagap was as mild as windless air, and when the breath of heat met the rime, so that it melted and dripped, life was quickened from the yeast-drops, by the power of that which sent the heat, and became a man's form. And that man is named Ymir, but the Rime-Giants call him Aurgelmir; [...]
The Ash is greatest of all trees and best: its limbs spread out over all the world and stand above heaven. Three roots of the tree uphold it and stand exceeding broad: one is among the Æsir; another among the Rime-Giants, in that place where aforetime was the Yawning Void; the third stands over Niflheim, and under that root is Hvergelmir, and Nídhöggr gnaws the root from below.
Hel he cast into Niflheim, and gave to her power over nine worlds, to apportion all abodes among those that were sent to her: that is, men dead of sickness or of old age. She has great possessions there; her walls are exceeding high and her gates great.Hel thus became the mistress of the world of those dead in disease and old age. One last mention of Niflheimr appears where it is the last destination of the jotun who were killed by Thor after he had built Asgard:
Now that the Æsir saw surely that the hill-giant was come thither, they did not regard their oaths reverently, but called on Thor, who came as quickly. And straightway the hammer Mjöllnir was raised aloft; he paid the wright's wage, and not with the sun and the moon. Nay, he even denied him dwelling in Jötunheim, and struck but the one first blow, so that his skull was burst into small crumbs, and sent him down below under Niflhel [Niflheim].