Norse mythology

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

Parts of Norse mythology included the great ash tree Yggdrasil, Hvergelmir, a well of poison (which turned out to be a natural well of poisons located at Maiden's Point) and the Great Serpent, prophecied to rise from the sea and spew venom over all the Earth. Fenric took advantage of these myths, planning to have the Great Serpent Ingiger take the poison from Maiden's Point and carry it to the sea to poison the world forever. Fenric's name, though not his real one, and his "wolves" were also based on the Ragnarok myth. (DW: The Curse of Fenric)

The Garm guarded the Forbidden Zone on Terminus, working in part with the Vanir. (DW: Terminus)
 * The was a dog that guarded, while the  were gods who raged war against the .

The artificial intelligence WOTAN (Vo-tan) shared its name with the High German god, who corresponded with the Norse god.

Anthony Rupert Hemmings believed that the Timewyrm was one of the Norse gods. (NA: Timewyrm: Revelation)

While crossing a stream of mercury, the Seventh Doctor compared it to, the stream that flowed through. (NA: Cat's Cradle: Time's Crucible)

Behind the scenes

 * The name Fenric is derived from the name of the wolf.
 * The Gods of Ragnarok's name is from the Nordic, which is often misunderstood as the "End of the World" or Armageddon. While an important part of Ragnarök is a climactic and cataclysmic battle, it ultimately ends with the renewal of the world as new and fertile.
 * One translation of Ragnarök is "the End of the Gods"; thus; the Gods of Ragnarok are "The Gods of the End of the Gods". This could be interpreted either as an added layer of meaning, or as nonsense.