Niflheim is the Norse mythological location where unworthy Vikings are sent when they die, and possibly those who didn't die in battle. It is mentioned in the Race to the Edge episode "Night of the Hunters, Part 1".
Niflheim (pronounced “NIF-el-hame;”) comes from Old Norse Niflheimr, meaning “World of Fog”. It is one of the Nine Worlds of Norse mythology and is overtaken by primordial darkness, cold, mist, and ice. It is often mistaken for Helheim, which is ruled by Loki's goddess daughter, Hel.
The word “Niflheim” is often used interchangeably with “Niflhel,” a poetic adaptation of “Hel", the world of the dead of which was named after the goddess Hel. The term “Niflhel” is found in Old Norse poems.
The well of Hvergelmir, from which all the rivers of earth flow, is said to originate in Niflheim.