Niflheim is one of the Nine Realms.
Millennia ago, the Asgardian king Odin banished his daughter Hela to Hel, a region of Niflheim, to quash her ambitious plotting to conquer the Nine Realms. Hela attempted to escape her prison, forcing Odin to dispatch a group of female Asgardian warriors known as Valkyries to fight her back. However, Hela successfully massacred all the Valkyrie, except for their leader, Brunhilde. As a result, Odin traveled to Hel, where he personally engaged Hela in a battle, ending with him re-imprisoning her, binding her imprisonment to his lifeforce so that she could not escape as long as he lived.
Four years later, Odin's life had come to an end as he died in front of his two sons. This in turn freed Hela from her prison in Niflheim, as Odin's life was no longer present to keep her trapped in the realm.
- In the non-canon video game Thor: God of Thunder, Niflheim is a base and thrall world of the Frost Giants. It is home to the Cave of Ages, where Odin imprisoned Mangog. Niflheim is noted for being the location of Hel, a dark region where the inhabitants of the Nine Realms apparently dwell after death.
- In Norse Mythology, Niflheim is the primordial realm of mist, ice, and cold whose rime flows combined with the flames of Muspelheim to form water which the gods used to create all of Midgard.