Princess Celestia: Destroy the tyrant's rule, we must. Shining Armor: Send me to kill the King. I will not kill Twilight. Princess Celestia: To fight this King Sombra, strong enough, you are not. Shining Armor: But Twilight...she is like my sister. I cannot do it. Princess Celestia: Twisted by the dark magic, young Sparkle has become. The sister you raised, gone, she is, consumed by Darklight Eclipse. Shining Armor: I do not know where the tyrant has sent her. I don't know where to look. Princess Celestia: Use your feelings, Shining Armor, and find her, you will.
Twilight, whatever good you have left in you, I just want to tell you I TOLD YOU SO!!!!!!!!! I TOLD YOU THAT PLAYING WITH DARK MAGIC WOULD CORRUPT YOU!!!!!!!!!!!! I WAS RIGHT!!!!!!!!!!!! YOU WERE WRONG!!!!!!!!!!!! NOW YOU"RE EVIL BECAUSE YOU DIDN'T LISTEN!!!!!!!!!
King Sombra: "Twilight, come to the dark side of the force!" Twilight: "Okay!" King Sombra: "Wait... seriously? I thought it would take much more convincing." Twilight: "Well nopony else seems to care about me." King Sombra: "Oh, you want me to care about you?! Never mind then."
Today I did some independent study of magics beyond that of friendship. They’re so much easier and more efficient! Why didn’t you ever tell me what kind of power could’ve been at my command all this time? I’ve seen you use dark magic before, Princess. I know you’ve experienced it. But tell me, do you know what it feels like to be… attuned to it? I don’t think I need you any more. I have a better teacher now. By the time you get this, I’ll be on my way to Canterlot. I’ve got some questions about what else you’ve been hiding from me, and I eagerly look forward to our conversation.
>But we all already know she'll become an Alicorn and replace Celestia. -Huhhh... it IS a theory, but not exactly a very likely one, if we are honest about it.
She's being ''trained'', she's very *knowledgeable* and more-and-more powerful with her magic, and at the end of the S3-EP2 there's that book with *stars and swirls*... which make people think of 'Star Swirl the Bearded'.
A slightly more realistic theory would be that she's being ''trained''/prepared to become the next ''arch-mage'' of Equestria, for the Princesses. Or, something along those lines.