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.

I have no idea how long I've been here, this gap between the seconds. It could be minutes, days, ages, I can't tell. I can never tell.

Maybe that's for the best.

The sun never rises, but it never sets. The clock never ticks, and I never rest. I never hunger, nor thirst. I certainly never tire.

It makes it much harder to tell the time I spend here...

Yet it never feels like none at all...

We all know the story of how the Princess’ young student and five friends defeated Nightmare Moon and brought Princess Luna back to our lands. But what if the tale were false? What if the young Twilight Sparkle on the event of her entrance exam had not been rescued by Princess Celestia, but instead lost her mind among the power of her youthful surge and had to be sent away, far away before she could destroy all of Canterlot with her newfound power. Now the feral unicorn prowling the Everfree Forest only vaguely remembers her past, slipping away from constant terrifying attempts to capture her.

And she is beginning to remember what true power felt like.

Editors include: Peter, Justin, Featherprop, Mitch H, and Bad Horse
Featured on Equestria Daily
Reviewed by Chris on One Man's Pony Ramblings and by Paul Asaran on his review blog
Now with a TV Tropes page

Physical books are available on Lulu.com
Monster in the Twilight (Paperback) 272 pages
Monster in the Twilight (Hardback)