Here question for anyone wondering on how would Celestia change into her Nightmare form. I'm asking this because of a trading card that her Nightmare is base off of. The Nightmare Star card.
And yes I do believe it non-cannon I'm just asking what would it take for her to change. I believed it would be doubt and mistrust that would change her.

The nobels/population manage to get Luna to leave Equestria, heartbroken and full of despair.

4492681 The inhabitants of Equestria neglecting harmony and thinking nothing of her. OR. She ends up being forgotten and left behind.... like Luna.

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I think it would evolve from a need to protect that just got out of hand. Maybe something happens to Twilight or Luna and she thinks 'never again' and so it slowly devolves into tyranny under 'mother knows best'.

Actually now that I think about it, I don't think there has ever been a story where the position of Nightmare is reversed for the two Alicorn sisters.

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Please tell me, this means Celestia is going to "Nightmare" in the show!

Also, I think a deep betrayal or a massive slaughter of her citizens would cause her to change. Nightmare Moon wanted her existence acknowledge. So, it seems, that Nightmare Star would want her rule to be absolute.

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I agree with this. Her only real road to evil is of the "Greater Good" variety in which she sees herself as protecting society from itself and gradually loses touch with reality.

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One of my favourite short stories in this site deals with that.
"Let's Just Say..." is a story about Celestia wondering if its worth it to keep Equestria alive if it means for her to have to deal with the upper nobility, and then specifically thinking about leaving the sun overhead and disappearing until all life baked to death.

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And this other short story deals with Celestia being a nightmare that just pretends to be a pony:
Cold Hearted

So let's see the current ideas of an Evil Celestia.

First off is a kinda rip-off version of Luna/Nightmare Moon.

Next we have the comic version of an Evil Celestia. Literally an opposite worlds type of scenario in which this Celestia is a Tyrant.

Then lastly is a fan idea. This is Celestia in what most fans think she would turn out to be. Nightmare took her form from the jealousy reflected by her sister living in her shadow. Granting her mane and tail elements of the power she commands. I believe so should Celestia with the fire along this one's mane and tail. I currently use this type of Celestia in my own Story "The Legends of Celestria."

"Daymare! Fighter of the Nightmare! Champion of the sun! You're a master of karate, and friendship, for everyone!"

She'd turn if she forgets how to do karate. Because the Nightmare would offer her the best karate lessons, and Celestia would fall into temptation.

After ruling "alone" for 1000 years, more than threatening her subjects, what could change Celestia in a Nightmare like way would be threatening\hurting Luna, Twilight or maybe even Cadence. Such a long time living ageless among common ponies the fear of being alone again could be too much for her to bear. I could see Celestia paying any cost to keep her "family" alive and with her, even against their wishes.
For example if ascension doesn't mean becoming ageless I could see Celestia ritually and periodically sacrificing half of Equestria to keep Twilight young and at her side, even despite her protests. Or maybe forcing Twilight into going Nightmare herself if it could extend somehow her lifespan and damn the consequences.
And now I have another idea for a story I could never write. The whole studying friendship is a load of bull, after seeing Twilight wanting to leave her side for her friends Celestia keeps sending Twilight and her friends to dangerous situations in the hope of getting everypony but Twilight killed, the whole ticket fiasco wasn't a mistake, it was a plot to try and separate her from her friends, the same with keeping Twilight from telling her friends about the alicorn magic and Tirek, this time trying to break the frienship with secrets. Sending her sister to Ponyville during Nightmare Night hoping to guilt-trip Twilight making her see Luna needed a friend more than her friends. The whole show since the third episode nothing more than one convoluted plot after another to try and separate Twilight and her friends to get her back to Canterlot in a way it couldn't be traced to Celestia ruining her friendship or killing her friends. And now I can't help but overthink this idea, what if the wings Twilight gave Rarity were outright dispelled by Celestia hoping to make Twilight unable to live in the same town as the pony she helped kill, the whole event with the false phoenix death, faking defeat at the wedding hopping the changeling would kill her friends retrieving the elements without a key... The more I think about this the more it seems like a good author could make a good and believable take on it.

4492681 the only thing I can think of is if Princess Celestia absorbed the 3 Dominus Glyphs

1. Dominus Hatred

2. Dominus anger

3. Dominus agony

They can easily consume the mind of the weilder

Simple put Celestia in a situation that she has no hope in where no matter what she will lose make her desperate for a way out crush her hope as it appears, make sure she always has something more to lose each time, then she will be willing to embrace Nightmare. Cannon Celestia is in control during the show when a huge threat arises and she can't do it alone she has the elements or some way to win, take that away and she is one step closer to giving in.