Niflheim is the underworld, land of the dead and one of the nine worlds of Yggdrasil. Ruled by the dark queen Hel, Niflheim and Asgard are at a state of constant tension as each manipulates the dwellers of Midgard for their own purposes. Demon Sword "Levantine" is Niflheim's sacred treasure though it was eaten by the dragon Bloodbane. As time does not flow for undead, it is likely time does not flow in Niflheim.
The Mists of Niflheim was a period of time on Midgard in which the undead invaded and waged war against humans.