From the Source:





Dear Princess Celestia,



Today I learned a very valuable lesson about friendship. Friends are easily decieved and abused. And so are you, Princess.



You have fulfilled your purpose in granting me the powers of an alicorn, but now you are nothing more than a hindrance to my studies. You have expended your usefulness to me, and as such I need you no longer.



What exactly am I talking about, you may wonder? It's simple, really. You thought me the most talented unicorn in Equestria, but have you ever wondered why is that? There are powers lying beyond your grasp, Princess, powers that you'd call forbidden and dangerous. Powers that have found me worthy the day you met me, powers I have studied for years. And now, thanks to your 'blessing', I shall wield them boundlessly.



The final summoning is soon to be complete, and once it is done you will hear of me no more. I can hear their whispers in my head, they are calling to me, they want me to unlock their final secrets… and I am more than willing to answer their call.



Take heed, Princess. Dark clouds gather upon the horizon.



Your former student, Twilight Sparkle.