Norse mythology

Norse mythology was the pre-Christian pagan religion of the Vikings in Scandinavia (Norway, Iceland and Sweden).

When Ace and the Doctor were at Maiden's Point, the Doctor refered to Yggdrasil and the well of Hvergelmir. (DW: The Curse of Fenric)

Behind the scenes

 * The name Fenric is very similar to the evil wolf Fenrir.
 * The Gods of Ragnarok's name is very similar to the Nordic Ragnarök, which is the End of the World or Armageddon.