Hey guys, OmegaBrony here for the first time.
I personally believe that the Pony of Shadows will be the villain of MLP Season 5. Why? At first I didn't know what to think of this shadow pony, but after watching intently and piecing a few facts together; I think I may have figured it out. Now bare with me, I am new to this.
In the very first season and episodes, MLP shows Nightmare Moon returning to bring eternal night. This got me to thinking why she became evil in the first place. Celestia said she became resentful and angry at ponies shunning the night, but I think Celestia may not be telling the whole story.
It seems Celestia, for the good of her subjects, keeps many ancient legends a secret; such as the Crystal Empire. Just an example, but even she told Twilight that the empire held a powerful magic and that it affected Equestria dramatically. She must have kept this from them to protect them from the possible fear of Sombra returning.
This made me curious, and for a long time, the questions stayed unanswered... until I watched the Season 4 premiere. When Twilight drinks the potion and goes into the past for the second time; she watches Celestia and Luna find the Tree of Harmony. Luna asks her sister if it is safe to remove the elements and Celestia replies, "Even without the elements, the Tree of Harmony possesses a powerful magic. As long as that magic remains, it will continue to contain all that grows here."
I didn't catch it immediately, but once Castle Mane-ia came out, I realized that the Castle of the Two Sisters was built directly over the Tree of Harmony. I figured Celestia and Luna did so to make sure they protected the area and could return the elements if they needed to. But it became evident that shortly after this, Discord was banished, and then, the change happened to Luna. When I looked at this chain of events, I subconsciously began thinking of how it was all connected, since in this show there always is a connection. And after the Season 4 finale, I finished it. So here it is:
I believe that the Pony of Shadows is not a leftover magic from Nightmare, but in fact an ancient evil that once spread darkness over Equestria, long before Celestia or Luna ruled or perhaps were even born. During this darkness, I believe the Tree of Harmony was somehow grown at its exact place to, as Celestia said; "Contain all that grows here." The Everfree Forest must be part of the darkness that the Tree of Harmony is suppressing. That explains why when the tree was in danger from the plunder seeds that the forest began expanding. True, the seeds themselves were expanding with it, but while the tree was weak, everything began falling apart.
Now here comes the big gamble in my theory. I believe that when Celestia and Luna built their castle over the Tree of Harmony, that darkness, or the Pony of Shadows, began to corrupt Luna since it could only come out at night for some reason, and maybe since Celestia was the patron of light, it had no effect on her at first. Then once Celestia banished Luna, she figured out that the darkness there corrupted her, and so she abandoned the castle without taking ANYTHING with her, including the elements; I think she was too afraid that the darkness would corrupt her as well. This also explains why Celestia, in the Season 1 premiere, did not appear in the castle until the sun was allowed to rise. I believe something is going to happen that breaks that darkness free. And I even have an idea on how that is going to happen, but I would rather wait to expand on that until I get a little feedback; if anyone sees a flaw in my theory, please, tell me. I am excited as the next brony to see what happens next.