Chrysalis has heard rumors of a crystal mirror having the power to take ponies to other worlds, where they gain powers that put them on par with the likes of Princess Celestia or even the Spirit of Disharmony himself. She wishes to use the mirror for her own ends, so she can gain the power to finally have her revenge on the ponies that have taken everything from her and maybe regain her empire in the process. The world she finds, however, is the dark interior of a ship that is being overrun by dead creatures that will stop at nothing to kill her and the repair crew that came to fix the ship.

She must quickly get used to her new body while fighting the hordes of the dead, come to terms with all of the horrible decisions she has made over the years, and maybe see the light that the ponies talk about... provided that she, and her new companions, don't get killed first.

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

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)

Jak never found it odd that he and his sister Pinkie were different from everyone in their village, despite the fact that their friend Daxter pointed it out from time to time. Nor did to the trio find it odd that Jak and Pinkie were raised by Samos the Sage, along with his daughter Keira. Jak never questioned how odd their life was, even when he and Pinkie slowly developed the skills necessary to protect themselves from any dangers the world might throw at them.

Pinkie, on the other hand, knows that there are secrets hidden deep in their past... secrets that will change how she and her brother look at the world around them, while fighting to protect it from those that would seek to do it harm.

(Jak and Daxter crossover)
(Part of the Universe 13 story series, though reading the other stories is not required)

Beerus and his attendant Whis find two young girls who don't know anything about themselves, save for their names. They quickly discover that the two girls aren't from their universe and, with the help of some of their 'friends', raise them with the intention of sending them back to their own universe one day. Until that day comes the two girls are stuck with Beerus, Whis, and their friends.

Applejack and Rainbow Dash will have to help save the universe from evil tyrants, genetic monsters, and a creature that should have been forgotten... while fighting along side some of the universe's greatest heroes.

(Dragonball Z/ MLP crossover)
(Part of the Universe 13 story series, though reading the other stories is not required)