Oddly, the nine worlds of Norse mythology were the first things that came to mind when I read Valhalla. I did consider Hel/Niflheimr but the dark elves demesne was what I went with. That the next response was the Jötnar's (giants) demesne made sense to me. Other names such as Yggdrasil or Ratatoskr would have equally acceptable (IMNSHO)
Mr. Spock