Twilight of the Gods
Deep beneath the earth and far to the north lies Niflheim, the realm that contains Helheim and all the courts and servants of Hel, goddess of death. It is also the dwelling-place of many monsters, including Nidhogg, the terrible worm that gnaws at the roots of the World Tree.
Niflheim is also the realm of the Rime-Giants, the largest and oldest of the Jotun, known as Hrimthurs. They do not answer to Hel and her minions, nor necessarily to Surt and his. They are ancient enough to remember that the world as it is now was created from the corpse of their father, slain by the Aesir. Only the god Thor dares to venture into their realm.
Niflheim is a Demi Plane connected to the Elemental Plane of Negative Energy. It is also connected to the Prime Material Plane by way of the Plane of Shadow. All the dead who are not brought elsewhere by one of the gods come here to await the end of the world.