I see Nightmare Moon as something Luna created. She was a friend who was there for her when Celestia wasn’t, and she took the form of what Luna thought she wanted to be, a version of herself who at least be respected if not loved.

@ThatGuyImpious  
Celestia never said Nightmare Moon was a separate entity. It was shown by making her distinct from Luna.

Ia Ia Nightmare fhtagn.

@DatImpyGuy  
Exactly my headcannon!

I think Celestia explained away Nightmare Moon as a separate entity for Luna’s sake.
 
It wouldn’t do for ponies to know the truth. The Elements didn’t destroy Nightmare Moon. Luna is Nightmare Moon, and simply had all of the power she had accumulated drained from her, allowing her to be calmed down and made to see reason. She is still spiteful and resentful, but having seen the truth of her actions - and being welcomed by a far more accepting society - have quelled the most dangerous of her emotions.
 
Luna is still one of the last ponies you’d want to fuck with, though. She’s hardcore old-school, back in her day they crucified dissenters. Maybe. Probably.

You have to acknowledge all the parts of yourself, even the parts you don’t like, because leaving those parts ignored leaves them free to cause trouble.

@Sjogre: I agree with your view in that I feel that possession would be a cop-out. At the same time though, I think it would be interesting if there was an episode in which lighter and darker sides of Luna are split into separate entities, and it’s up to Twilight and Co to bring them back together- a fate neither side wants. In the end, there’s a lesson about….something.

Sometimes, when Princess Luna felt truly alone and frightened, SHE would come to her in her dreams.  
SHE had a coat as dark as the space between the stars, with a mane made of moonlight and star-fire and lightning-flame. SHE was clad in armor forged from lunar steel, with wings that crested from horizon to horizon. SHE was the thing ponies feared when they shut their eyes, the shadows on the walls, the whisper in the midnight wind that rattled the branches of Everfree.  
“Come be free,” SHE would whisper to the Princess. “Come with me and be free. Would you come with me?”  
And one night, Princess Luna, unloved and forlorn, whispered her answer.  
“Yes.”

@ClownDicks:
 
ASSUMING DIRECT CONTROL

@ClownDicks
 
We know you feel this Sparkle.

^My guess? It’s not so much possession as it is Reaper-style indoctrination.

I’ll admit, I think that it would be a better story if Nightmare Moon was something Luna did to herself. It adds more to the character if she has to live with her mistakes.
 
Possession just feels like a cop out to me. Nothing she did as Nightmare Moon would have been her responsibility, so it doesn’t really affect her character. She’s just another victim, which I find less interesting on both a character and plot level.
 
Having someone else use the magic that Luna used to turn herself into Nightmare Moon would also allow for pretty much all of the plot hooks that Possession would, with the addition of whatever drove the character to do that to themselves in the first place.

Ok then. Hopefully season 3 will shed some light on it.
 
Personally though I think it would be more interesting if it was a separate entity.

Nope, nothing concrete.

Yeah I read that but I meant something more concrete.

V Lauren gave hints, but nothing’s been said concrete so far.
 
Let’s hope Season 3 gives us some answers.

So was it ever determined whether Nightmare Moon was a separate entity or just an alternate personality of Luna?