I saw this comment on this article that states that there's a rumor on how the era before the avatar will be explained in Book 2 of Korra. It goes like this:
"This is a rumor of the plot of season 2, thousands and thousands of years ago before the time of the Avatar a great 300 year war took place between the Spirit World and the Physical World. Each side wanted to control more of the other side, to keep peace between both the Avatar was created as a bridge between both. This also allowed the Avatar to keep peace in the Physical World as well as the Spiritual World. The Dark Spirit or Spirit you see in that photo is one of the original Spirits of the 300 year war that took place thousands of years ago and they are finally returning to sieze the world."
What do you think guys? Well, I think it's a good theory. :D