Niflheim is a primordial realm in Norse mythology, often associated with cold, darkness, and mist. It is one of the nine worlds, serving as a counterpart to the fiery realm of Muspelheim. Niflheim is significant as it represents the origins of the cosmos and is home to the well of Hvergelmir, which is a source of various rivers that flow through the world, tying it to creation and the cycle of life and death.