Niflheim is one of the Nine Realms.
Millenniums ago, the Asgardian king Odin banished his daughter Hela to Hel, a specific region of Niflheim, due to her violent ambitions to conqueror the Nine Realms. However, Hela attempted to escape her banishment from Niflheim, forcing Odin to dispatch a group of Asgardian female 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 confronted and engaged Hela in a battle, which ended with him re-imprisoning her and using his life as a lock to her imprisonment in Niflheim.
A few years after the Convergence, 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.