King Sombra’s dark influence was obliterated by the Crystal Heart, and the villain is well and truly dead. But when he attempts to steal the afterlife of an innocent victim, a human spirit is left hurtling towards Equestria, where this unfortunate soul is revived into the only vessel available: the reconstituted body of King Sombra himself.

Thus, a cheerfully oblivious alien is crammed into the form of the most evil and malicious being Equestria has ever known. Not that Shining Armor and Cadence know this, of course. As far as they are concerned, the King is back and he’s lost his fricking mind.

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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

In an effort to find the couple that truly needs her help to find some romance in their life, Cadance develops a spell to send her to them.

It's just that when she gets there, neither of the creatures seem that interested in her help. They want to talk about silly stuff like her magic, the existence of other dimensions, stupid government conspiracies meant to keep aliens under wraps, and whether she is a winged unicorn, or a pegasus with a horn when they should just KISS ALREADY!

After the Friendship Games, Sunset feels it's time to return home.

Only when she gets there, Twilight's castle is nowhere to be found, Cadance just got her horn, and Princess Celestia is still calling the bacon-haired pony her prized student rather than a failed one. And just reliving her past isn't on the table thanks to a pair of wings Sunset seems to have sprouted upon her return to Equestria.

With Sunset being back before she's supposed to, will the world stay as harmonious and on track, or will everything go down in flames as Sunset has to deal with allegations that she's really a changeling in disguise, a nerdy stallion with blue hair that wants to be her coltfriend, a pretty pink princess in need of remedial magic lessons, and an annoying purple pre-teen pony that will absolutely not go to bed without first being read Starswirl's Complete Mystical Mysteries Compendium?

Only time will tell

Once upon a time, on the not-so-magical planet Earth, there was a US Marine who signed up to defend his country. He was assigned to a fire team, the fire team assigned to a squad and the squad assigned to a platoon. Together they defended the area surrounding a Forward Operating Base in Afghanistan from insurgents. If only it were that simple.

When lone Marine is transported to Equestria he finds a post-apocalyptic wasteland. With no orders, no resupply and seemingly no hope of return, he sets out to find what brought him here, and why.

Strong Language Warning: He swears like a Marine.
Disclaimer: I'm not a Marine

What do readers have to say about this story?

"[T]his has to be the dumbest sounding plot I've ever heard of. But my god do you take that and fuck it in its ass ... Its just so real and witty and genuine that the whole premise actually works!" ComprehensiveBrony

"Yes, it's human-in-Equestria, but it's good. ... the action scenes had me sitting on the edge of my seat" Tebee

Cover by me, background stock by DarkDragon774

In the frozen north of the Equestrian Wasteland lays a city, battered and crippled, but still clinging to life. It is beset on all sides my dangers.

Displaced griffins rage against the ponies they blame for the apocalypse. Madmares brood with their unquestioning minions under mountains of prewar technology. The Unity extends its malefic tendrils to sake it endless hunger for slaves. Unnatural horrors stalk the forests, consuming all in their path. A dozen factions rage against each other in wars of words, caps, secrets and fire, tearing at the very foundation of The Sovereign City. All the while an ancient threat prepares to once again, bathe its self in the blood and anguish of ponies.

Welcome to Flankorage.

(Comments are encouraged as this is my first attempt at fan fiction. The more feed back I can get, the better I can write.)

Having lost much of her past, memories and purpose in life, Aurora Borealis stumbles upon the junkyard and its sole inhabitant Scraps. Having regained her name and desiring to find her past and place in the world, she begins to walk the paths of discovery. But what she finds and what impact she has may be more than she ever expected.

SF (canon species only; no humans) Ponies were magically and genetically engineered to be the perfect servitor race. They are powerful, adaptable, intelligent and completely under the control of their creators. A laboratory accident frees one such pony from her mental chains, but how can one mare save herself and the rest of her kind if she doesn't even know she's a slave?
This is not the Equestria you know and these are not your little ponies... not yet, anyway.

Tags: Adventure, Diamond Dogs, Gryphons, NO HUMANS (did I mention that already?)
Other tags: historical, mind control, Clarke's 3rd Law, excessive use of high energy physics, before they were famous.
Detailed review by PaulAsaran.
Cover art by endrome , used with permission.
Russian translation (partial -- 'unofficially' completed in the comments)
A Soot-Covered World, book 1.