That, or some crazy pony starts to believe that she's Luna, and goes on a rampage trying to figure out what happened to her horn and/or wings and/or why her cutie mark is wrong.
Luna slowly regained consciousness. She was laying in what appeared to be a workshop of some sort, surrounded by bits of metal, with a large blacksmiths hammer clutched in one hoof. A large glowing forge dominated one end of the space, and she could feel its sweltering heat from across the room.
Stumbling onto all four hooves, she tried to flap her wings for balance. They didn't respond. In fact, she couldn't even feel them at all.
She attempted a minor diagnostic spell, only to have the magic slip quietly out of her reach, almost like it had once when her horn had been... snapped off.
She quickly pressed a hoof to her forehead, feeling for the telltale jagged stump that she expected.
There was only a smooth expanse of fur and skin.
Something was terribly wrong. Alicorn horns could heal, but this was obviously something else entirely.
With a sense of muted horror, she craned her neck around to look at her flank.
A blacksmith's hammer, the spitting image of the one that she woke up with, stood bare and stark on her flank like a brand of damnation, without even a wing obscuring its mocking countenance.
A familiar voice sounded in the hallway. Even in her shocked state, Luna knew exactly who it was. It was that thing that wore her face, her body like nothing more than a fleshy mask. Nightmare Moon had returned.
Shoving away her terror in some deep recess, Luna strode to the doorway and pushed it open. She had a duty to perform, even if she had somehow been twisted by magics most foul.
It was worse than she had expected. Nightmare Moon had split herself into dozens of copies. Somehow, they had not noticed her foalish move, and she backed into the workshop again, closing the door quietly behind her.
Luna did what she should have done in the first place, reaching out with her spirit into the aether, and crying out to her sister.
There was no answer.
Mercilessly, she grabbed on to the primary solar ley line, and traced it end to end.
Nothing.
Celestia was gone.
A strangled sob escaped her throat, but she cut it short. She was a—the princess of Equestria, and she had a duty to her ponies.
Having created the all the little ponies with the help of her sister, she had a bond with each and every one of them. It wasn't something that could be affected by mere flesh and blood, or even the demonic energies of the beast that lurked about outside the blacksmith's workshop.
As she had tried with Celestia, she reached out, trying to find anypony other than the monster. Further and further she reached, encompassing the entire world with her spiritual search.
There was nothing.
Nopony was left.
“All my little ponies, dead and gone. My Sister, taken from me! Nightmare Moon, you will PAY in BLOOD!” Luna screamed in rage, picking up the blacksmith's hammer that she had awoken with.
She kicked open the door, and surprised the first Nightmare clone she came across, crushing its skull with a mighty blow.
Clone after clone fell to her mighty rage, but Luna was still encased in the body of a mortal earth pony. More and more of the monsters came, and she found herself struggling, despite the fact that in the confusion some of the Nightmares seemed to have turned on each other.
She had lost count of how many she had slaughtered, but they still kept coming. She was covered in wounds, and bleeding out.
She was going to die.
Luna was going to die, and Nightmare Moon would be left to rule a stolen realm.
She shook her head, being given a brief respite as a nearby clone went mad and attacked its fellows.
No. Luna was going to die, and so was Nightmare Moon.
With a peace she hadn't felt for millennia, she reached once more into the aether, and gave an almighty pull. Two crescent moons flickered briefly into existence, obscuring the bloody hammers on her flanks.
Having dispatched the aberrant clone, the monsters closed in on Luna. She simply smiled sadly at them.
A spear ran through her chest and an axe lodged in her side.
“I'm sor-ry Celly,” she gasped. “I love you.”
The moon crashed into the plane, followed shortly by the sun.
The Endless Fields, plane, has crumbled to its end.