It's been a hundred years since Twilight was crowned. Most of her original friends are gone. Now, only Pinkie Pie and Fluttershy remain, each with a spark of something special keeping them past their time.

But unlike Twilight, neither of them has a claim to true immortality.

Winner of the 15th Quills and Sofas Speedwriting contest. The prompt was "A Worthy End." The contest was a two-parter, and each chapter was written in an hour, and then expanded slightly afterwards.

This story is a sequel to How Not To Use Your Royal Prerogative

  In an Equestria where the Sonic Rainboom never happened, Sunset Shimmer is the prized pupil of the Princess of the Sun. As Sunset struggles to make up for her past misdeeds and come to terms with the pony she wants to be, Celestia sends her to find the classic work The Application of Unified Harmony Magic from the Royal Canterlot Archives.
  Right after lunch, some meddling by her friends and meeting somepony with firsthoof experience of the Sunset she used to be.

Featured on Equestria Daily on August 18, 2016!
Appeared in the FimFiction Featured Box!

Cast: Sunset Shimmer, Moon Dancer, Minuette, Twilight Sparkle & Princess Celestia

Stories set in the Wavelengths Timeline in chronological order:

Origins Arc
The Alchemy of Chemistry
Bards of the Badlands
Grading on a Bell Curve
Habits of the Equestrian Phoenix
How Not To Use Your Royal Prerogative

Applications Arc
The Application of Unified Harmony Magics
Princess Celestia: A Brief History
The Cloudsdale Report

Dreamers Arc
Tactics of Snowbound Unicorns
A Study in Chaos Theory
Teahouses of Saddle Arabia
As the Raven Flies

Cover Credits
Cover Design by Amber Spark
Sunset Shimmer Vector by TigerBeetle
Pony SciTwi Vector by Osipush
Canterlot Bookshelves Vector by FavoriteArtMan
Sunset Shimmer Cutie Mark by MillennialDan

Beta Reader Credits
Ebon Quill - Worldbuilding Writer on The Manehattan Project
Quantum State - Technical Director of The Manehattan Project
The Albinocorn - Master of Sunset Shimmer Stories

Strongly Recommended Stories:
The Brightest and the Best by Pineta - The events in this story are considered to be completely canon to this timeline and explain Twilight's entrance exam to Celestia's School for Gifted Unicorns. I cannot recommend this story enough.
MLP Annual 2013 - "The Fall of Sunset Shimmer" - As above, the 8-page short comic in the beginning of the Annual is also considered to be canon, though with some major variations.

Version 3.0 Edit: Extensive grammatical and mechanics changes. Adjustment to timeline of Sunset's second mirror experience.

This story is a sequel to A Stitch in Time

Having discovered the message left for her by Star Swirl the Bearded in the distant past, Twilight once again finds herself caught up in the middle of a struggle against forces far greater than herself. In order to even stand a chance she'll need to travel further and wider than she ever has, and forge some unlikely alliances with old enemies. All to bring down the powers that have manipulated and shaped her entire life, and retake control of her own future.

The conclusion of the Time Loop Trilogy, which began with Hard Reset
Available in Spanish thanks to dgs1993
Series TV Tropes page

For almost a thousand years, the immortal Nightmare Moon has reigned over Equestria as Empress. To the average pony, Nightmare Moon is an overarching enigma whose rule is absolute and whose wrath is terrible. But to Twilight Sparkle, who as an orphan has never known her parents, the Empress is something else entirely. When the young unicorn is brought under the royal wing as an apprentice, she learns a great deal more about the supposed tyrant than she ever knew. But as Twilight is introduced to life with her new mentor, she is thrust into a world of political intrigue, conspiracies and secrets. Secrets that were never meant to be revealed. As allies and enemies plot against the very peace of the nation, will Twilight fall prey to the perils of her new position? Or will she prosper and find in Nightmare Moon the mother she never had?

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My eternal gratitude to my editor, LordOfTheWrongs, without whom Night's Favoured Child would not be the story it is today.

Now with added TVtropes!

Princess Twilight Sparkle discovers a centuries-old mystery hidden in the Royal Archives. Her investigation leads through layer after layer of deception and misdirection, setting her hooves on a path that seems to be leading to a mysterious secret. Along the way, she learns that some friendships can be very, very strange.

Book 1 of the Alicorn Adventure series.

AUDIOBOOK by VisualPony
Featured at the Royal Canterlot Library
TV Tropes Page (Courtesy of Sandtalon)
Goodreads Page (Courtesy of Facedeer)
Equestria Daily Page

WARNING: Major spoilers in the comments. Takes place several months after the end of season four. (Not a Da Vinci Code crossover, BTW!)

Do you see a dark tag? No, you don't. Not dark. Well, okay, there are a couple of dark-ish bits, but that's all. Not enough to warrant a tag. Speaking of tags, there really ought to be "Badass Twilight" and "Friendshipping" ones.

Sequels and side-stories (and more) can be found here Offprint and AO3, and a hardbound two-volume set of all that can be ordered from The Ministry of Image.

My editor for the first edition of this story was Gogito, and I can't thank him enough for all the hard work he did. My editor for the revised version was PresentPerfect, and he did a terrific job of getting the story into excellent shape.

Long ago, the founders of Equestria built a magical portal on sacred ground, hiding their greatest secret on the other side. Generations of leaders stepped through the portal so that they might emerge as the equal of their forebears.

Celestia and Luna were the last to enter. For a thousand years, the portal lay forgotten, its purpose lost.

Luna thinks it is time that Twilight learn what it means to rule.

There is now a Spanish translation of this story, courtesy of Spaniard Kiwi

Celestia has been waiting for this night her entire life; Twilight Sparkle has been destined to come for years upon decades upon centuries. But she didn't know to expect her student like this.

She steels herself anyway. She pours some tea, and welcomes Twilight with open hooves.

Then the two of them have a talk.

Rainbow Dash is not afraid of monsters.

But maybe she should.

Live reading by Solar Pony

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Experimental fic, a mixture between words and pictures. Not your usual narrative.

Princess Celestia always knew that Twilight Sparkle was destined for great things, but she never revealed the source of that knowledge, not even to those closest to her. When the day of Twilight's true destiny finally arrives, can Celestia fulfill an ancient promise to save Equestia's past, present, and future, even it means saying goodbye to Twilight forever? How can Celestia refuse her mother's last wish?

Art by the inimitable Harwick

Celestia and Luna have lived long and suffered greatly. Luna, still insecure in her newfound grace, is anxious to secure herself against any further suffering. She sees in Twilight Sparkle (and in many others) the roots of ten thousand tragedies. Celestia proposes a wager.

A very loose retelling of the Book of Job.