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.