[This letter is written in a somewhat archaic style, and appears to be very old. The integrity of the paper and ink has been maintained over hundreds of years by magic even older, though its wards are now failing.]
I cannot imagine the pain in her heart.
One thousand years of twisting the strands of fate into her own tapestry, what she wanted the future to be. She knew the time was coming, yet the vessels of the elements of harmony had yet to be even born, and her sister's darkness took greater hold of her every day... so when the time finally came, she ripped the elements of harmony from the fabric-that-is-yet-to-be, and with them wove anew.
It is fortunate they both agreed afterwards that it was the right thing to do. I'm certainly glad for it, even though the first time I heard this tale I was shocked that my great and benevolent princess could do such a thing.
But I digress. These words do not flow here to speak of history, though they record a personal side of the story known to few. I write these words as a testament. To whom? Alas, I do not know. Perhaps when I am done I shall burn this paper to ashes, to float upon the seaward wind and into the blessed night...
A testament to myself then. For now at least.
Top of the class, I was. Still am, though it means nothing. The Princess of the Sun's prized, most faithful student. Envy of all young scholarly ponies. A bookworm, anti-socialite, and totally friendless. Though I dislike saying it, I didn't really consider my assistant to be more than a glorified messenger until after I met the other vessels and learned the magic of friendship. But I'm getting ahead of myself again. Suffice to say I was as introverted as a Klein bottle, and leave it at that.
So of course, when I connected the dots between the legend of the moon with a little known prophecy, all the arrogance of youth drove me to believe the Princess would heed my warning. Instead, she packed me off to the small town where the Summer Sun Celebration was scheduled to be held and told me to make some friends. It is there, in that town where I met the other vessels my best friends (is this what I've come to? It's been so long since I've thought of them as much else... Even my truest friends are echos to me now...) I still wonder how she guided that book into my hooves... Artful manipulator she was. Is. (Will be?)
[There are tear stains on the paper here, smudging a few of the words slightly]
This is going to be harder than I thought. For my own sake I best get on with it.
The rest of story, at least the official version (which is close enough), is known far and wide in Equestria, and I needn't go into the details. I was there, after all. Even looking back, I still don't wish I knew all the implications of those events as I do now. All that freedom, the time with my friends, all our adventures together in that small town at the edge of the Ever-Free Forest... I wouldn't change one quantum of it.
Oh, I'm dithering again. Time grows shorter still, and I want there to be some earthly record of my memories, even if there is nopony who will ever read it.
Here I write something almost nopony knows: The Elements of Harmony aren't those stone balls in the grand hall. They aren't the jewelry we wear. It's all a facade. We don't just wield the elements of harmony--
We are the elements of harmony. The physical incarnations thereof. In our own ways, we were are as powerful as the Princesses, perhaps more so. That night, the night of the thousandth Summer Sun Celebration, our souls and the spirits of the elements of harmony melded, and we became more powerful than any pony imaginable...
[A break in the writing is made for tear-smudged splotches indicative of a broken nib. The writer appears to have pressed too hard upon the page, and destroyed a feather pen]
I'm glad it never occurred to me exactly what it meant when the Princess took me on as her student, as her protege. Back then, in my youthful arrogance I dismissed it as a sign of her favor, of my superiority in studying and learning. Stars help me, I actually thought it was just because I was an excellent student!
To some degree, I suppose it was. However, I think it was mostly a convenient cover for her real purpose: Molding me into a pony worthy of my current position and title... Molding me into the Regent of the Sun and Moon. Fitting, isn't it? The vessel of the element of Magic, regent of all of Equestria, ruling in the stead of the Princesses after they ascended. Ruling until the spirits of the Sun and Moon are again reincarnated. Ruling, until now.
For a thousand years, I have done it. I have seen Equestria into a golden age. A golden age of laughter, loyalty, honesty, generosity, kindness, and magic. How I wish I could have shared all those moments with my friends...
Ah! Another pang. Very soon, I shall have to release the element of magic from my being. For a thousand years it has been a part of me, but nopony can hold such power in check for more than a thousand years. It yearns to be free, and I must release it. My friends have all done this already with theirs, and in doing so ascended beyond this mortal coil. For five hundred years I have yearned to join them, but I could not. My promise to the princesses binds me to this plane of existence until the Sun and Moon are born and christened anew. As such, the timing of tomorrow's christening of the royal foals could not be better. After the christening, I shall seal myself in the chamber I have prepared and meditate from high noon until the final strike of midnight...
[The author appears to have left off writing for a while and taken up the pen again some time later, as the final lines are written in a different color ink.]
I know to whom I write now. Your name came to me as I dreamt last night... floated into my aged head on an ethereal breeze. I write to you now. I know you will find this. I shall hide it where I know only I, and thus only you shall find it.
I, [the signature here is too faded to make out], write this testament of myself, my past, and what I believe to be the future, to the next incarnation of the Element of Magic--
I write this testament to you, Twilight Sparkle.