Fallout Equestria: The Ashlands Timeline

If you have seen Friendship is Magic through season 5, you don't need to read this part, just skip to the first chapter. It's just a summary of what happened up to that point to avoid some confusion if someone hasn't seen the show to season 5.

Season 1
They established that Rainbow Dash’s first sonic rainboom caused the mane cast to get their cutie marks, binding them by destiny to become the Elements of Harmony, which allow them to defeat stuff in-show. The first one they defeated was a possessed version of Princess Luna named Nightmare Moon who wanted eternal night.

Season 5 Premier
They stopped a plot by Starlight to replace all cutie marks with equal signs to make them equal. She escaped. They acquired a ‘cutie map’ table-like device that lit up when it had a friendship mission.

Season 5 Finale
Starlight attempted revenge by going back in time to prevent the sonic rainboom that brought them together. She did this by altering an ancient spell by Star Swirl the Bearded.

Twilight followed her back. Every time she failed to stop her, it created a new future due to the friends not having come together. There was one where Nightmare Moon took over and caused eternal night, one where Chrysalis’s changeling hive took over, as well as others.

Finally Twilight dragged Starlight forward to see the future she was creating in an effort to convince her to stop. They ended up in what the fandom calls ‘The Ashlands Timeline’. That is where this story starts.

Comics:
In brief, this states that Nightmare Moon came about due to the Nightmare Force possessing her, and verified they can possess others. The Nightmare Force is native to the Moon and is led by ‘Larry’.