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.
Cover art by Swirling Line.
