As hinted by the title, I tend to believe there are certain parallels or even influences with/from Norse mythology, especially Ragnarok, in the last few episodes of Book two of LoK.

Take a close look on the final battle of Book two, between Korra and the Unavaatu. Now... let's just say that we take the Avatar universe Earth as Midgard AKA the ancient Norse term for Earth. Then the Spirit World would be a combination of the 9 realms in Norse mythology, like Asgard, Vanaheim, Hel, Jotunheim, Muspelheim, Nieflheim, Nidavellir, Svartalfheim, Alfheim, with the two spirit world portals
form the Bifrost bridge. The Tree of Time which Vaatu was imprisoned in would take the place of Yggdrasil, the tree connecting all Nine Worlds.

Korra and her allies would form the Aesir, the Norse gods, to some extent at least. Korra herself would be a female version of a combination of Odin, Thor, and a son of Odin's named Vidarr. The reasons: Odin: because he is the highest of all gods and stuff, Thor: for slaying Jormungandr, the serpent holding Midgard in its place, and last but not least, Vidarr: for slaying Fenrir the wolf who kills Odin and swallows the sun and moon. Now, to proceed with the antagonists...

The Dark Spirits would represent the Giants and all others who would oppose the Aesir and their allies. Unavaatu, the Dark Avatar, o whichever you prefer, however, would be a different story. Vaatu and Unalaq would be a union of Jormungandr, Fenrir, and Loki. Reason being is that Jormungandr and is slayed by Thor, Fenrir slays Odin who is slayed by Odin's son Vidarr, and Loki because it is Loki who starts Ragnarok and because like Unalaq is a trickster and like Vaatu is a being of chaos. Unalaq/Vaatu "slayed" Korra in a sense, destroying the Avatar cycle, BUT was defeated by her as well.

So, we could say that the final battle of Book two was indeed Ragnarok, the Norse Doomsday. And LIKE Ragnarok the end was just the beginning as a few gods survive and rebuild the world, much like how the world was forever changed by Korra leaving the spirit portals opened.

In conclusion......... VIKINGS FOR THE WIN!!!!!!!

[youtube=www.youtube.com/watch?v=3qlI6wBUQdk]