From what was seen in the show, we know that just like how Luna has known as Nightmare Moon, Celestia also has a dark side known as Daybreaker. However, the only appearance we ever see that form is in the episode, A Royal Problem, and she only appears in the dream realm. Other than that, we never see or hear about Daybreaker ever again in the show. After hearing that, it would be interesting to know what later episodes would have been like if Daybreaker made an actual appearance and how it could possibly impact the show itself.

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There was one other appearance of Daybreaker, which also occurred in a dream state (induced by King Sombra), in "The Beginning of the End":

Interestingly, this seems to imply that either King Sombra or Twilight are familiar with Daybreaker via some other means, since neither of them were involved in the dream in "A Royal Problem".

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Twilight probably knew about DayBreaker via her... 'interview' with Starlight at the end of 'A Royal Problem'. The scene showed a very giddy Twilight dragging off Starlight to ask her about her 'Harmony Tree' mission.

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However, the purpose of this thread is to discuss what it would be like if Celestia actually became Daybreaker and the Mane 6 had to actually fight her just like they did with Luna when she became Nightmare Moon.

The closest we get is in the comics. Both the Reflections arc and the Nightmare Knights arc. Spoilers for both if you haven't read it yet! I might be remembering some parts wrong, too.

I can only assume the one in the Reflections arc could have been Daybreaker once. She had been around long enough that that could have been a "character growth" version of Daybreaker. Season 3 Alicorn Twilight looks much different when compared to later and final seasons. Or she just like the Somnambula style of dressing.

As for the Nightmare Knights arc, Daybreaker doesn't really have anything going for her near the end. No personal goal or any true desires yet. It is only after her closing remarks that we get a sense of a phyrric victory. She wants to see the Casino grow, but in her own villainous image.

With that as the base, it is likely the mane 6 needs an equalizer if they really can stand a chance. Daybreaker probably won't go "scorch the planet route" if she got something to gain with a very much alive population. We're still missing details from the very first two parter of the series: where did Celestia go?

So I'll leave this speculation as to how the mane 6 could deal with Daybreaker: Celestia has her prophecy, but Luna will always be her counterpart. She could exhaust Daybreaker enough that the mane 6 could tackle her on like Nightmare Moon while she recovers her magic.

After all, Nightmare Moon didn't really kill anyone directly with magic. Especially when she charged at Twilight instead of shooting a beam like we see in her battle with Celestia. Nightmare Moon probably exhausted herself with her sister.

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Interestingly, this seems to imply that either King Sombra or Twilight are familiar with Daybreaker via some other means, since neither of them were involved in the dream in "A Royal Problem".

Ooh, good point!

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I think of Daybreaker as being a fictional creature invented by Starlight, given she's literally a NMM palette swap and behaves nothing like Celestia. However, I do think a proper Nightmare Celestia is possible. Maybe by corruption from an outside force, maybe if she felt like her little ponies had lost their faith in her and didn't want anything to do with her anymore.

In the case Celestia did start to become corrupted, I think the show would handle it by having Twilight talk her down. The two are extremely close, and Twilight always believes in Celestia and sees the good in her, and I think that like she and Starlight did with the Pony of Shadows and her and the girls did with Luna and the Tantabus, she could enter Daybreaker's mindscape and find Celestia within her.

I mean I could give a whole episode idea for how that would shake out, but that's my impression of what would happen in the show if Daybreaker/Nightmare Celestia showed up for real.

I know that if Daybreaker was to make a real appearance in the show, Twilight wouldn't want to fight her at first especially since she doesn't want to confront her own mentor.

I wrote a story on this a while back, Daybreaker, in which I explored the possibility that Daybreaker's appearance in Starlight's dream was actually a memory, not by Starlight, but by Celestia herself. That in the distant past, before even Luna's transformation to Nightmare Moon, Celestia became Daybreaker. Which is why Celestia looks regretful when Daybreaker appears in the dream realm, and why Celestia's final words in fending Daybreaker off are "... you will never exist again!" Starlight was worried about Celestia turning power-mad and evil of course, but that might have prompted Celestia's recollection of a time when she actually did. And since Celestia was unfamiliar with dream-realm powers and how to keep her own personal issues out of the picture, it caused Starlight's fears to manifest in the form of Celestia's earlier self.

On learning about all this from Starlight, Twilight in the present day (being Twilight) simply has to find a way to go back in time and learn how it is that Daybreaker isn't still ruling Equestria, since that era has been carefully scrubbed from the history books. And in the process Twilight does have to deal with the fact that her beloved mentor isn't the perfect Princess that Twilight has always believed she is.

And of course Starlight and Trixie go along for the ride, because Twilight needs Starlight's help with the time-travel spell, and of course bringing those two along could never lead to any time-travel-related issues, right?

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I actually like how the episode pretty much stated that Daybreaker is never going to happen. All she'll exist as is as a fear within Twilight and Starlight's minds, or as a little temptation in the back of Celestia's head...that she'll soundly ignore.

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I do plan on doing a fanfic in the near future that will involve the Mane 6 and Megan fighting Daybreaker, but this all I'm going to say right now, because I'll be giving it away if I say anything more, and I don't like giving spoilers.

While we are mentioning fanfics, there is a brief mention of Daybreaker twice in Luna Comes Clean, which I did with the first one being where Celestia gave her side of how she viewed Luna as Nightmare Moon to Megan with the other where she also says it to come clean in the big announcement in concurrence to Luna to doing for herself.

I am actually working on an Egyptian/cyberpunk themed setting where instead of Nightmare Night returning, Celestia gradually became Daybreaker and it wasn't until after she had scorched much of Equestria that she was sealed away.

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Daybreaker is real through and through.