Before we begin, i want to make this clear; i do not have all the facts yet, but i will try my best to tell my theory; so lets begin!

_______________________________________

"There is a way to release ourselves. A spell to obtain our souls once more. We must speak it; but when we speak, no sound. We are all dumb. So failure is inevitable. And I regret ever entering the eternal bet with the devil inside me."

These words are a poem which i took from a game and rewrote it for luna's theory, so this will have much more impact if i explain:

Luna has always looked up to big sister, but when it became nighttime; she got no attention from anypony. This reason is because all ponies are sleep while she watches over their dreams, but she wasn't satisfied with this at all; she wanted all ponies to see her work. So she secretly makes a deal, she asks the darkness to grant her wish; but it would only do it on one condition, she must give up the one thing she values most; her soul. With this agreement, the darkness takes her for its own.

This is the point of no return, the deal has been made and Luna loses everything she is; and becomes that darkness. Why? She was willing to give anything to have her night sky noticed and nothing will stop her from doing it. We've seen how determined Luna is at making friends, what makes you think she wouldn't give in to weakness?

But Luna realizes all too late that this darkness doesn't want to help her, it wants to get rid of Celestia and bring forth the eternal night. All Luna can do now is watch as this thing attacks her sister and fails; both Luna and the darkness are banished to the moon for 1000 years. In that span of time, Luna regrets her deal with this darkness(Which i refer to as the devil inside), and wants it all to end. But whenever she speaks; her voice is no longer there; she can't speak.

Now here's the point where i stop, Luna has made a grave error and cannot take it back; her soul is trapped and used by the darkness to fuel its magic, this in turn helps it gain the power to rival Celestia's own magic. But there is a catch, while it does have Luna under it control; there are times where Luna tries to call out; but cannot.

This is because her soul holds her voice, and her life; but this darkness can manipulate it for its own personal gain, so any attempt to speak is futile. So the darkness speaks for her, but not the words she wishes to be heard; only words of jealousy and hate come out. This is her sin, and she now must live with it until somepony can save her.

So when the elements of harmony are used on her, her soul is freed and she returns back to normal once more. Luna learns from her mistake and never does it again, favoring her abilities and friendships over greed.

______________________________

So ends my theory, if you have any questions, please feel free to ask.