It all started with a simple question: What if? What if Luna had ponies who noticed the beautiful nights she created. Would she have turned into Nightmare Moon? Would she and Celestia rule together for a thousand years? Would Twilight meet the rest of the bearers of the elements and become friends with them? Now, with the help of a new magic spell that she created, Twilight is going to find out!

Welcome to the Lunar Republic!

Featured 4/9/15

Celestia is not invincible. Strong, yes. Ageless, assuredly. Magical, undoubtedly. But not invincible.

The Princess of the Day has been attacked, and she did not escape unscathed.

While Luna fights against the mortal fear gripping her subjects in the wake of the assassination attempt, the enemy runs free. Twilight Sparkle must assemble a team to bring the pony to justice. Few ponies can be trusted, and fewer still can be expected to succeed.

But the kingdom has heroes: The Knights of Harmony. Ponies who have stood against evil and won, who fight to make Equestria a safer place. Each is an expert in their particular field, masters of their trades, but they have yet to work together.

An adventurer past her prime.

A tinkerer with too much time on his hooves.

A spy whose primary instinct is to lie.

A dishonored soldier with an axe to grind.

As they delve into the origins of the assassin, they find that Equestrian history still has secrets long buried. Can the knights come together as a team before she finds her next mark?

This is a fic based on the Time Loop Files, the format of which was first codified by Innortal on fanfiction.net.
Here's a brief guide.
The basic idea is, one character (the Anchor, usually the main character of a fictional setting) is looping back to the very first moments of their parent series every time either they die or some kind of time limit expires. They of course end up monumentally stir crazy given enough time.
Eventually others start to loop as well.
The reset is not always perfect. Sometimes a loop's history will be different to the "prime" loop, or canonical plot.
"Crossover" or "Fusion" loops also occur, randomly. These can involve the home loopers having a guest, or the anchor for one universe spending time in another, or replacements of one character by another.
Vacation Loops are where the Anchor (or others) decide/s "buck it" and lets off steam by doing whatever comes to mind. There's little or no attempt to maintain the original timeline.

In addition, and particularly more recently, it should be known that this story has a collaborative element to it - people may submit loop ideas or loops, ideally at the Spacebattles thread (a link to the current version of which is on the Trope page), and they may become included.

There's two page image options, thanks to Filraen - the current page image has been edited by the original creator from this work of theirs.. The other one is here:
http://hourglass-vectors.deviantart.com/art/MLP-Loops-466030169