This story is a sequel to Fading Moonlight

Thousands of years after Luna's sacrifice to Tirek, Celestia and Twilight are the only two remaining in Equestria. But even they are destined to die, as the end of the world nears. Shortly before the last day of their lives comes to an end, they reminisce about their most memorable experiences in Equestria, and recall the events of the night Luna died. Soon after, an emotional farewell takes place.

Rated Teen for literally the slightest death reference ever (and the fact that it's kind of the point of this story).

Understanding of Fading Moonlight is not necessary unless you want to know how Luna died.

Also, Twilight's 5 friends, Spike, and Luna only appear in the epilogue.

When Celestia is weakened following the wrath of Tirek, Luna goes to extreme measures to save her from further torment. She'd do anything to protect her sister, even if it meant giving her own life.

Rated Teen for what I'd consider a reference to suicide (self-sacrifice), and, of course, death.

It's been nearly a decade since the Battle of the Bands, but Adagio's guilt hasn't left her yet. Sunset is able to get her through it, and just hours later, has a very important question for her beloved.

(Rated Teen for a few profane words.)

After Luna and the citizens of Ponyville defeat the Tantabus, Twilight writes a letter to Celestia regarding the events, including an explanation of why the Tantabus came to be. Celestia is devastated at this news, and decides to confront her sister. Little does Luna know that she's not the only one facing so much guilt.
Based on Season 5, Episode 13. Told from Celestia's point of view.
(Also, rated Teen because I put, like, 3 cuss words in it.)

Celestia is once again forced to endure the Summer Sun Celebration, for the 990th year in a row. Realizing that while she may be just 10 more years away, it'll still be a short while before she can see her sister again, she finally gives in. Realizing that she's been content for too long, she finally decides to seclude herself from the rest of Equestria, finding out that almost a thousand years is far longer than she can endure, and the time for her to let it out has finally arrived. However, she finds some comfort with the help of a little filly.