Discord has been telling Rarity about the various adventures that her friends are supposed to be going on, in 'other worlds' no less, though she politely listens to them so she doesn't hurt Discord's feelings. She never once believed that he was telling the truth, not with how outlandish some of his tales had been, but her mind changes when Sunset Shimmer and Starlight Glimmer return from their own adventure with powers beyond anything she had ever heard of before. Before long she decides to investigate the mirror herself, where she steps into an unfamiliar world that is unlike anything she has seen before... only it is a world that terrifies her.

Rarity awakens in a place called 'Things Betwixt', a limbo between worlds, where she discovers that she's in an unfamiliar body and in an unfamiliar world with a different set of rules than what she is used to. Along the way she discovers that she has been cursed in some manner, making her one of the cursed undead that roam the new world she has discovered, and that there might be some small hope of reversing her condition. She also discovers an companion who wishes the same thing and decides that, by working together, the two of them can free themselves from their curse and return to their normal lives.

She and her companion must survive the dangers of the world she has found herself in, complete the most insane quest she has ever heard of, and hopefully cure themselves of their curse... and hopefully she can find a way back home once their quest is over.

(Dark Souls 2/MLP Crossover)
(Part of the Universe 13 story series, though reading the other stories is not required)