This story is a sequel to Split Second

Twelve years ago, she broke time. Two alternate timelines exist, each with a different version of Twilight Sparkle. Both versions of her are able to see the other. In this timeline, Sparkle is a Necromancer.

Or rather, she was. Sparkle is dead now, not that she cares. She's got a new body. With Thorn (her undead dragon son) and Cobalt (her apprentice and an assassin for hire) at her side, she's ready to take on the world. Again.

(Spoilers in the comments. Read at your own risk.)

Featured on EQD.
The Tv Tropes page
The Ask Blog

This story is a sequel to Split Second and won't make sense without it.
This story also loosely shares continuity with Lupine Tree. Reading that is not required.

Cover art by Swirling Line.

Twilight Sparkle broke time when she got her cutie mark. Now Twilight and Sparkle are sisters from another time. Twilight is hailed as the potentially greatest light magician of all time, and already has a reputation as a hero by the time she goes to Ponyville. This isn't her story, not primarily.

Sparkle is an orphan - her dark magic killed her parents. She lives with her brother in Canterlot's industrial sector because they can't afford anything better. She can't leave either. She's a natural dark mage; while Sparkle won't go mad like Sombra, her reputation is hardly any better. Worse, the law doesn't see her as a good pony.

The fact that the dead don't stay dead around her, or that Thorn - Spike's double - is an undead, soul-eating abomination, are other matters altogether.

(Also has some Slice-of-Life, but fimfiction tags won't let me put it with Adventure.)

Has an ask blog to go with it: Ask Sparkle and Thorn.
Also has a TvTropes page: Split Second Tropes.

Featured on EQD.

Cover art by Swirling Line.

After discovering that Cadence is not who she appears to be, Twilight is sent to another world entirely by the disguised Changeling Queen to avoid her plan being disrupted.
When Twilight lands, she discovers that not only have Spike, Owlowiscious and Peewee followed her, but that they've arrived in a world where the main sentient species is human.
There's still a place where they teach magic, though.
Crossover with Harry Potter. Twilight will be effectively taking the role of Hermione.

I've marked this story as cancelled to reflect the unlikelihood of it continuing. If I do come back to it I'll revert that.

Bored one evening, Discord suggests to Celestia that they spice things up by seeing who's best at doing the other's job: he'll handle keeping things harmonious, and she'll be there to throw a big ol' spoonful of chaos into the mix whenever she feels like it. To his surprise, she agrees to do it. He's certain he's got this one in the bag—after all, how hard can something as boring as harmony be?

He shouldn't have underestimated his opponent.

UPDATE: Featured 2-15-14!
—————
For the Writer's Training Ground #007 down at Equestria Daily. Originally posted on my DA account, due to timing issues.
Cover Art and MLP:FiM in general © Hasbro/Lauren Faust

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