Niflheim is an optional and unlockable location in God of War and is one of the Nine Realms of Norse Mythology. Niflheim is known as known as "The Realm of Fog", due to the cursed mist that plagues the land there.
A major feature of this realm is a vast labyrinth filled with traps and enemies. Located further into the realm is Ivalidi's Workshop, which contains many treasures, some of which can only be obtained using unique currency of that realm called Mist Echoes. Furthermore, the workshop houses three Realm Tears.
Deep in the labyrinth is the Valkyrie, Hildr.