Twilight of the Gods
Nithavellir is the Hall or Refuge maintained by Sindri, a dvergar master crafter who is said to have forged weapons and armor for the gods and heroes of Valhalla. The hall is under the earth, but little else is known about it. It is a single hall, but can be reached from many different places, and a path that led to it at one point will be a dead end the next time one tries.
Presumably, if you find the Hall of Spark, there is a chance that you will meet the gods there from time to time.