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 Left Behind

After defeating the Broken Pony, Nightmare Moon is finally free to begin her plans to retake Equestria and restore it to its former glory. However, she is haunted by the ill portents she had been given and searches for signs of her sisters' fragments or the coming of any new alicorns.

On top of this, the king impedes the alicorn's work at every turn and tries to be rid of her just as frequently. So many things happening all at once and spiraling towards the doom the 'leftover tool' warned her about, but Nightmare Moon is determined. No creature will stop her goal, not even the dregs of Equestria's destroyers from so long ago.

Luna and Celestia are not like you and I, or at least that is what we tell ourselves. They do not talk to those such as us, who are mere mortals who quickly pass on by them. At least, that's what we tell ourselves. Until one day, my Lady spoke to me.

I will never forget that night. Even though I want to forget it.

Edited by the ever lovely MaskedFerret.

Twilight Sparkle has walked the wastelands for a very long time, but now, at last, he has reached the end of his journey. He only hopes now that the gods will listen to his prayers, and finally allow him a good place to die.

(Please read the story before commenting on the description)

Puzzle Piece is the premier information broker in the infamous nation of Freeport, leading one of the most powerful and wealthy organizations in the islands. However, after months of absence from Freeport, he returns to find everything he built in shambles and his lieutenants fighting over the remnants. With chaos reigning in the streets, the Council tells him to get his house in order or see his life’s work swept away. Unfortunately for Puzzle, there are forces in Freeport that don’t want to see him succeed. Nobody can be trusted, nothing is safe—not even his deepest secrets.

A chronological sequel to Northern Venture.

Sequel: Left Behind: Sunny Disposition
Apparently it was scouted and is on EqD too

Nightmare Moon was meant to be released after a thousand years. A momentous occasion for her but terrifying for all the ponies on her day of freedom. She would get revenge on her sister and cover the world in darkness and shape everything to fit it.

That was the plan, at least. Instead, something went awry with the spell and she wasn't released when she was supposed to be. The mare-in-the-moon is brought into a world with stagnated technology, giant cities, and no sign of her sister or any of the other immortals. In fact, ponies don't seem terrified of her at all, and the stay within the core of her powers infused and changed the alicorn's body drastically.

The alicorn had no obstacles left! But...she was also a relic of the past. A past everyone seemed to have forgotten. A past she could no longer look upon.

Artwork by ElkaArt. Link in source.
Big thanks to Kryostase for helping spot errors I've missed.

It’s not every day you discover that your friend is in an adventure novel, but that’s exactly the situation Sunset Shimmer and Kukri Doo find themselves in when Puzzle Piece, Freeport’s foremost problem solver, makes an appearance in the latest Daring Do book. They only naturally ask Puzzle about it, and uncover a truth far stranger than fiction: a tale of adventure, intrigue, tyrant lizards, the mysterious dromaed people, and the fabled City of Giants.

A Freeport Venture prequel.

The depths of Luna's loneliness drive her to create a creature that never should have been. But the nature of her companion, and the dark methods used to create it, may lead Celestia to commit an unforgivable deed of her own...

Written for World-Building Alliance's January Writing Contest. Credit, as always, to NorsePony, for his wonderful editing work.

This story is a sequel to The Third Wheel

Double Time is a changeling. Years ago, she fought the Crystal Empire in the war in the north. Now she's Cadence's prisoner.

Together, they'll learn something new about life, and rulership, and what it means to love.

This story is a sequel to A Foreign Education

Twilight was sixteen when she became an alicorn. She hasn't aged a day since. Some ponies ask her what it's like, not being able to grow up.

Twilight can't really say. She's never known anything else.