Princess Luna is in a time loop, it can start from any point before Discord's first rule of Equestria and ends when Luna fails to defeat him a thousand years later (if she does defeat him, then the changelings, sombra or tirek bet her instead). Due the events leading up to nightmare moon do not happen in this universe, she and Celestia are still connected to the elements, so the loops consist of Luna trying to save Equestria and failing.

The only things that follow her through the loops is the pocket dimension that she has (everypony has one) and the stars, the spirits of ponies from days long past that see all, hear all and know all, often offer her advice in poetry format and are very selective about what they tell her (They know how to extend the length of the loops, they just don't tell her).

Feel free to post submissions for future loops.

Things of importance

-the royal canterlot voice is something Luna invents over the course of the loops as she starts going crazy and in a similar fashion to that one tortoise from 'the neverending story' "Started talking to ourselves".

-Discord's actions and the contents of Pinkie Pie's pocket dimension are randomised.

-any variations in a loop are caused vi a 'butterfly effect', Things Luna does can change the behavior of other characters and so on and while DIsocrd may leave a ton of references to stuff when he breaks loose, variations in a loop are strictly MLP -verse only, you could have Sunset shimmer succeed in taking over equestria, but nothing as bizarre as 'pokemon in equestria' or something.

You are of course, welcome to post loops based off of canon, as that is where this thing is headed. if you have any questions, feel free to ask.