From the files of Prof. Crimson Tome at Maneskatonic University:

It is with great trepidation that I take up my quill once more to add a new, and final, entry in a journal that I have not touched for longer than I care to remember. Yet now, in the twilight of my years, I feel compelled to explain – if only to some unknown reader – the reason for why I have never taken a student as my protégé save for one young stallion whom I retained many years ago. Perhaps I also feel the need to unburden myself of the terrible knowledge that I have steadfastly kept from all those around me. Perhaps, after I have written as much as my aged hoof can bear, my nights will no longer be haunted by visions of that unfathomable terror stalking the dark reaches of the Everfree.

Why does Princess Celestia leave Twilight to face all the villains?
Sunset Shimmer has returned to Equestria, to confront her former teacher.
The solar princess decides it's time to answer Sunset's questions.

Every so often, a storm comes along that even the Pegasi can't control. Best they can do is warn everypony to bunker down and ride it out. Equestria hasn't seen a blizzard this bad in over half a century. AJ and Big Mac were out when the first winds hit, so Applebloom's stuck with Granny Smith in the farmhouse.

But ol' Granny remembers, she tells Applebloom. She remembers the last blizzard, and what came with it. Can't help but wonder if they'll stop by again.

Sometime following Twilight's ascension to alicornhood, she goes through a personal revelation.

(Grimdark?)

Do you want to hear a scary story?

You do? Oh…well…okay. You’re the first pony I asked who didn’t turn me away. Most just looked at me funny, or just told me to get lost. A lot of ponies don’t like scary stories. I know I don’t.

What? Oh, I guess I should introduce myself. My name’s Scootaloo.

Chrysalis didn't invade Canterlot in a bid to take over Equestria. She attacked the city because she had nowhere else to run.

Now, imprisoned for her crimes deep under Canterlot Castle, she can sense the nightmare she fought so hard to escape from drawing ever closer. Soon, it will cross the Equestrian border and lay waste to Celestia's domain.

She must warn Celestia, before it's too late.

there is a moment

once in every pony's life

when the time comes

to shed one's baby tail

don't worry, sweetie

it won't hurt

Murder has come to Canterlot.

When a body is discovered, mutilated beyond recognition, Princess Luna is tasked with finding the culprit and bringing them to justice.

As the Alicorn of Darkness, it is her duty to watch over the night, and protect the ponies of Equestria from that which lurks in shadow. But she won't be alone in this endeavor; a stallion from the Royal Investigative Service and the prince of chaos himself have been sent to aide her in this matter.

Together this strange trio will find that there is much more at play here than mere murder; a storm is gathering in the shadows of Canterlot, and even the powers of a god may not be enough overcome it.

Prereading credits go to the eminent Carabas.

Twilight Sparkle is reading a book she can't seem to put down. It's not even a very good one, as the author keeps on writing directly to the alicorn. For three days now this is all she has done, as her friends desperately try and find her. Oddly, the author of the book knows more about Twilight than most... even if their only goal appears to be her death.

My name is Twilight Sparkle, and I woke up today to find that everypony I have ever known has vanished. I'm alone in the world.

Help me.

Audio reading by Crafty Arts.
Audio reading by Monanniverse.
Audio reading by GutiuSerenade.