Luna has had enough of Celestia's games, and has found the perfect means of revenge. Using a long-forbidden spell, she transforms her sister into little more than a small filly, all so she could pull the greatest prank ever. Unfortunately, she soon learns that before you fool around with dangerous magic, you really need to read the fine print...

A sequel can be found here.

When Twilight plays with magic beyond her control, she finds herself and Rainbow stuck in a strange, horrible world. When they find out what happened, will they be able to get home? Will they even want to?

This story is a sequel to Off The Edge Of The Map

When Twilight is sent with Luna on a diplomatic mission to Draconia, they quickly find themselves somewhere that doesn't appear on any map, and Luna is lost in more than one way...

Thanks to RBDash47 of Pony Fiction Archive for formatting it for submission.
Title image by Valcron

Tremendous thanks to Illya Leonov and Scribbler for the Audiobook version.
Now with a Spanssh translation by SPANIARD KIWI: https://www.deviantart.com/spaniard-kiwi/art/Apoteosis-2-3-857110089

Twilight Sparkle, Celestia and Luna are transported into a strange alternate Equestria, the Land of Always Summer, where the day lasts forever and a terrible queen rules with an iron hoof.

Feel free to check out or add to the story's TV Tropes page.

Twilight's curiosity pushes her to go where no bookworm has gone before: on dates. Dates with lots of unfamiliar (and not so unfamiliar) mares. It goes about as well as you'd expect.

A parody of dating websites in general, named after eHarmony.

A diplomatic crisis arises between Equestria and the small, poor kingdom of Tarandroland, ancestral home of the reindeer. Celestia and Luna agree that it is time Luna takes a more active hand in foreign relations again, and Luna brings Twilight with her as someone she actually trusts and can talk to. But the mission turns out to be much more than political discussions - if scheming unicorn courtiers, unfriendly surly reindeer, bloodthirsty pirates, ancient sorcery and dangerous monsters weren't enough, our heroines must deal with something old - older than Luna. And is Luna really ready for diplomatic action, will Twilight learn to use a fan properly, and exactly how wrong are Spike's superhero comics about reindeer mythology?

Now has a TV tropes page

Twilight Sparkle is accepted into Princess Celestia's School for Gifted Unicorns and gets a little more than what she bargained for.

Illustrated by me.

(If you object to swearing, please skip chapter 6. That's the one chapter that bumps this story straight up to a Teen rating.)

Twilight becomes alicorn of the stars. This is sort of a problem, because Luna kind of already was alicorn of the stars. Oops!

TV Tropes

Banishing Queen Chrysalis was not without complications. Twilight has found herself on the plane of Innistrad, a dark place wrestling with ghosts, spirits, vampires, curses, and other things that should not be. Now she's on a world with no backup, no books and no plan beyond basic survival. Yet friendship is still a powerful thing, even in this new world of gothic horror. Now Twilight must find a way home, while still trying to do the right thing. [MLP:FiM][MtG:Innistrad block][Horror-genre themes]

When Ruby boards the first trans-oceanic flight in Equestrian history, the last place she expects to end up is stranded in the middle of Eternity's Crossing in the wake of a horrific zeppelin crash. After a mysterious tower leads her and a fellow survivor to discover a twisted and fragmented city twenty thousand feet below the surface of the ocean, though, a bit of fire and smoke is the last thing she needs to worry about. Between the horrendously disfigured residents and the paranoia of the madmare who once built and controlled it, daily survival in this corrupted utopia is anything but a guarantee. But with the help of some new friends and a few genetic enhancements, Ruby isn't going down without a fight, and in the process might just discover that the deadliest weapon in Harmony is herself.

An adaptation of BioShock for the world of MLP. Written to be comprehensible to anyone, regardless of whether they've played the original game.