Pandinus Imperator (You can call him Pan, he won't mind) is knocked back in time during the Canterlot Invasion, and accidentally kills King Sombra on arrival. Instead of being executed, the changeling is surprised to find himself being hailed as a hero, and is promptly put on the throne as the new monarch, the Crystal Emperor. However, Pan now faces a great many problems, among them Celestia and Luna, who seem certain that the changeling is secretly up to no good, as well as several political factions secretly plotting to undermine his reign. There's also the tiny problem of the constant snowstorm that threatens to cut the city off from the outside world permanently.
The Crystal Ponies need a hero now more than ever before. Why did they get a changeling instead?

We Has A TvTropes.

While not truly a part of Bucking Nonsense's Changeling-Verse, the same rules regarding changeling abilities and society apply.

Cover image provided by gabapple, used with permission.

This story is a sequel to Stardust

Times are changing for Equestria as Twilight’s return brings with it news of people and cultures from beyond the peaceful world everypony knows. Aid and volunteers are freely offered to help the besieged humans of Earth because every Equestrian knows that’s what good friends do. Unfortunately, making friends can also have the unintended consequence of making enemies.

When the monsters besieging the humans strike at Equestria with a level of brutality not seen in over a thousand years, the cry for help is sent to all the kingdoms of the world. Honored allies step forth to fulfill ancient treaties and oaths. Enemies, old and new, bury their grievances. An alliance never before seen in Equestria will be created to combat the monsters from the void.

And because of these events, a grim but welcome sibling to the XCOM project is born.

(XCOM/FiM Crossover)
(Sequel to Stardust, which I recommend reading before starting this. Fortitude Amicitia and Broken aren't required reading but are supplemental to the storyline)
(Gore tag for blood and violence)
(Alternate Universe tag because of the divergence from canon that Stardust has caused)
(Uses elements of Season 4)
(Coverart produced by the amazing Carnelian!)
(Many thanks to Arzoo, BradTheBrony, Vandenbz and Setokaiva for prereading!)
(As of 03/07/2016, the prereading team consists of Arzoo, BradTheBrony, Noble Cause and Metallusionsismagic!)
(Complete with it's own TVTropes page as part of the Stardust continuity. Could use some updating!)
(Character tags will be added as chapters are added!)
(Featured 5/12/2014! Thanks folks!)
(Possible spoilers in the comments! BEWARE!)

This story is a sequel to Past Sins

Side-story to Past Sins - Spike almost lost Twilight to a hangmare's noose, but now Twilight is safe and Nyx is a filly again. Normal life is returning. Still, Spike can't ignore the fact that Nyx just isn't the same as he remembers her. Her temper is more volatile, and, when she gets angry, all he can see is the rage-driven Nightmare Moon of legend.

And thus Spike has to wonder: Is Nyx truly reformed, or is she one bad tantrum away from calling herself Nightmare Moon again?

This story is a sequel to Project: Sunflower

After the events of Project: Sunflower, Erin Olsen has returned to Ponyville, happy to pursue her studies in magic. After everything that happened the year before, she's looking forward to spending her days studying and having fun with her friends.

Meanwhile, on the newly discovered world of Harmony, humans and ponies work together to study this strange new world. However, their arrival causes something ancient to begin stirring. Something that may end up changing everything.

Erin will need all of her courage, all of her cleverness and, of course, all of her friends to see herself through what happens next.

Cover art by Valkyrie-Girl
Editing provided by:
Brilliant Point, Coandco, Ekevoo, Ludicrous Lycan, Merlos the Mad and Razalon the Lizardman.

This story is a sequel to Past Sins

Hearth's Warming is a time for friends and family to come together, celebrate harmony, and welcome the new year. Nyx is looking forward to just this as she rides the train to Canterlot with Spike and Twilight. She's going to meet her extended family for the first time and wants nothing more than to make a good impression. Yet, the sheen of the holidays can't hide the trouble caused by meddling grandparents, relentless reporters, and a paranoid uncle.

On a dark night filled with even darker magics, an insane cult attempts to give Nightmare Moon a body and life of her own, utterly separate from Princess Luna. But, when the spell is interrupted by Celestia, something unexpected is created.

Now, a little black alicorn filly named Nyx finds herself living under the care of Twilight Sparkle, only to be haunted by memories and emotions from her past. Is she Nightmare Moon reborn or is she simply a doppelganger with a soul and mind of her own? Can Twilight Sparkle protect Nyx from those who refuse to see past her dragon eyes and black coat?

Or, will Nyx be forced to inherit the sins that may not even be hers and become the greatest evil Equestria has ever known?

This story is a sequel to Avocation

Life in Ponyville has been relatively quiet recently, which is somewhat surprising, given that the town's librarian tried to usurp the Equestrian throne only a few months ago. Aside from the appearance of a grumpy pegasus who set up shop as a cloudsmith, the aftermath is so unnoticeable that most ponies aren't even aware that the incident happened at all. Unfortunately, all that's about to change, thanks to the return of a certain revenge-obsessed showmare....

This is the fourth story in The Petriculture Cycle; reading the first three stories, which begin here, is highly recommended.

The artwork was provided by Page Turner, and the TV Tropes entry can be found here.

This story is a sequel to π

After Twilight Sparkle made her biggest secret public much earlier than she'd originally planned, the lives of the Element-Bearers and their closest friends somehow managed to become even more complicated than they already were. Yet despite the flurry of all of the added activity this brings, Princess Celestia decides to undertake a particularly ambitious project: the reformation of Discord. She delegates the task to Penumbra Noctis, who, more than anypony else, can empathize with the draconequus' situation, and who is therefore best suited to showing Discord the magic of friendship. On the surface, it's a very reasonable plan.

Unfortunately, Discord has never had much patience for reason.

This is the fifth and final story in The Petriculture Cycle. Reading the first four stories, which begin here, is highly recommended.

Artwork by Page Turner. TV Tropes page by an unknown number of anonymous persons on the internet.

This story is a sequel to Inscape

Penumbra seeks gainful employment. Hilarity ensues.

This is the third story in The Petriculture Cycle. The TV Tropes entry can be found here.

Artwork by Page Turner.

This story is a sequel to Petriculture

It's been several months since Pinkie Pie made a stunning admission about her past that permanently changed her relationship with her friends, particularly Twilight Sparkle, but life seems to have settled back to normal (well, normal by Pinkie Pie's standards, at least). However, when an old nemesis returns and ensnares Twilight in her influence, it's up to Pinkie to make things right.

This is the second story in The Petriculture Cycle. There is a TV Tropes Page for the entire Cycle here.

Cover image by Page Turner; "Princess Twilight" vector by Nianara.