Celestia asks Luna to teach Twilight dark magic. Twilight must pass Luna's class in order to graduate. As Luna develops feelings for her student, Twilight questions which princess she really wants to serve and protect.
(Twilight X Luna)

Since her return and rescue, Princess Luna has been looking for some way to properly honor the six mares who saved her from herself, but nothing she has been able to think of has seemed meaningful enough to repay her saviors.

Nothing, at least, until she spends Nightmare Night in Ponyville, and gets to actually know the six ponies to whom she owes such a great debt. She realizes she enjoys their company, and wishes for them to spend more time with her, that she might grow closer to them.

So, in accordance with many fine and ancient traditions, and after thinking it over for a good...couple of hours, easy, she proclaims that she shall marry all six of them. Oh, and that Twilight will be marrying Celestia, too.

Now would be the appropriate time for everypony to panic.

--Cover Art Courtesy of Zemious--
--Editing Graciously Provided By BronyWriter--

NB - The sex tag is in place for sexual situations and suggestions; the story is, and shall remain, free of explicit sex.

This story is a sequel to Becoming One With the Night

It was supposed to be a good day. Twilight and Luna were sharing the news of their relationship with their closest friends and family. However, Celestia's largest secret yet finally came out. Now, relations are strained between Celestia and everypony else. After all, what's Twilight to do when she finds out her second mother is actually her birth mother?

On top of this massive bombshell, soon Twilight will be sporting a pair of wings of her own. If she thought things were complicated now, just wait until Canterlot gets wind of a new alicorn. Life is about to become one giant nightmare for Twilight. But hey, at least she has Luna, right?

As stated above, this is a sequel. I highly encourage you to read the first story (the writing is extremely rough early on, mind you), but it's not entirely required to understand most everything that will be going on here.

EDIT: Takes place during S3, when it was originally conceived, thus the Alt. Universe tag. There was no redemption of Discord. There was no ascension. Yes, the Crystal Empire is back, and Cadance and Shining Armor are the leaders there. This should cover the more critical aspects of the season in regards to the story.

This story is a sequel to Apotheosis

Celestia leaves Equestria in the hooves of Luna and Twilight.

While she searches among the gods to answer her questions, Luna and Twilight must struggle to fulfill their own duties to ponies and to Equestria.
Now with a Spanish translation by SPANIARD KIWI: https://www.deviantart.com/spaniard-kiwi/art/Apoteosis-3-3-857110357

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 is called away to tend to Luna when she falls ill. As Luna grows sicker and Twilight more determined to help the younger princess, she begins to uncover dark secrets about the elements, the alicorns, and her own past.

It's the day of Hearth's Warming Eve. Twilight is preparing for the holiday when a surprise guest comes to her house. It turns out to be Princess Luna and she wants to spend Hearth's Warming Eve with Twilight and her friends. Twilight accepts and the two decide to have a slumber party! Little do the two of them know that a deep secret that both of them had will be revealed to each other. Will the secret bring them closer or tear them apart?

Twilight ventures out into the mountains to see a once-in-a-lifetime event. Disappointed by her friends not coming with her, Twilight crosses paths with a kindred spirit.

Celestia leaves to deal with a foreign crisis, giving her sister a chance to rule alone and become more attached to the modern world. She calls on Twilight Sparkle to advise Luna, hoping her student will be able to draw Luna out of her shell. With two major threats to Equestria already defeated in the past year, it seems like the perfect time to let Luna gain some confidence and experience in the modern world. After all, what are the chances of a third disaster occurring?

Now comes with complimentary tropes.

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