Two symbiotes, Nightmare and Shadow, reside in another dimension, waiting for the right pony to corrupt and transform into evil ponies of darkness, so that they may plunge another world of light into darkness.
An interpretation of an event that was long-speculated.
Update: I was just reminded of something suggesting that Nightmare Moon isn't like the Pony of Shadows in this way, that Luna became that way on her own, and that even Legends of Magic is non-canon to the show. So, I added an Alt. Universe tag.
Chapters (1)
You know, considering she was able to turn her form into Nightmare Moon on Nightmare Night, it's most likely not the same as the Shadow Pony. It really is just Luna throwing a temper tantrum.
The way the comics portray just seems off. The comics say Nightmare Moon came from when she was stuck on the moon, but the show has her becoming it before she ever went there. And I put the shows canon before the comics.
8662546
This is going by Legends of Magic comics. If any of the comics are canon to the show, it's them and the movie tie-ins. I agree that Nightmare Rarity, Siege of the Crystal Empire, etc. are non-canon (actually that was the "dragons aren't lavaproof" world I mentioned). Especially since Shadow Play portrays Starswirl banishing the sirens quite differently from the comics.
To the first point: noted, and I'll put an Alt. Universe tag on here. It really is much simpler viewing Nightmare Moon's defeat as 100% redemption anyway, confusing enough splitting Pony of Shadows between redemption (Stygian) and imprisonment (Shadow).
8662913
That's kinda the impression I got from Midnight Sparkle. Initially a corrupted form of Sci-Twi, once Sci-Twi is restored, the magic that made her Midnight Sparkle became an entity of its own, remaining inside her and waiting to control her again, until Sci-Twi made her the second of three main villains to die, and the only one of the three who isn't a king. (Because comic canon is separate from show canon, Sombra stays dead in the show)
8662546
The comics never showed that Nightmare Moon took control of Luna on the moon. Much as the show stated it, the Nightmare Rarity arc implied that Nightmare Moon came from inside Luna, a creature that was in her body long since and grew stronger because of her jealousy and hatred.
What came back from the moon and took Rarity was a mere remains of Nightmare Moon that stayed behind there, theories how that was possible differ.
Further confirmation that Nighmare Moon came from inside Luna and that Nightmare Moon is still sealed inside Luna that can be found in the official comics comes via the "Ponies of Dark Water" arc from 2016 and from the very first issue of "Legends of Magic".
The latter is not outright stated, but strongly implied, giving a hint how Nightmare Moon may have come into Luna's body a long time ago.
The comics are, and always have been, in complete alignment with the show continuity-wise. They have never provided a reason to believe something differently.
8663299
I thought in Nightmare Rarity, Nightmare originated as a creature from the moon and convinced her to let her in, like Rarity. I suppose they are a bit vague on the specifics of it.
Legends of Magic, which seems to fit more with the show, is what the Nightmare part of this story was based on.
Other than them and the movie tie-ins, the comics are inconsistent with the show, through dragons not being lava-proof, Amore having a different cutie mark, Sombra having a different personality, and different depictions of Starswirl banishing the sirens. Plus in To Where and Back Again, Chrysalis refers to A Canterlot Wedding (one princess) as "last time"
8663299
Certainly wished they touched Nightmare Moon more.
Like exploring what if Luna was meant to have those dark powers and such.
Exploring darkness and such.
Of course according to Jim Miller, Luna manifested Nightmare Moon from her own feelings of bitterness and resentment, which came out as a separate entity possessing her (but still believing herself Luna due to originating as part of her, similar to Evil Buu). Meaning no symbiotes involved with her, despite the similarities to the Pony of Shadows transformation.
Fans don't take his word as absolute canon but I could go with it.
10296250
So pretty much a split personality?
I wished they could have touched on that more and those inner dark powers and such.
10361353
Yeah, kinda like Dr. Jekyll and Mr. Hyde.