Sunset take lessons on how to control her demonic side?
Author's Note
Just a silly idea that came to me, hope you like it. (Feel free to make a sequel if you do.)
Constructive criticism welcome!
Know that I don’t have anyone to beta my story, and English isn’t my birth language.
The Only True Chapter
While, after taking a magical rainbow to the face, Sunset Shimmer had expected to be punished some more, she hadn’t expected to be stuck at school, in principal Celestia’s office, long after everyone else had gone home, in the company of said principal and her sister, vice-principal Luna.
"Miss Shimmer," finally said Celestia. "While the circumstances were not the sames, know you were not the first students to turn into a demon in this school. The last time, it happened while Luna and myself were still students." At those words, the principal nodded to her sister who, in front of sunset, rapidly turned into a demon.
Said demon was different from Sunset’s own corrupted form, with a skin as black as night, cat-like eyes that seemed to be turquoise in color, ethereal hairs of a midnight purple and full of gleaming stars, bat-like wings that seemed in a far better state than Sunset’s own had been, her whole body covered in armor of a slightly bluish metal.
"Vi–vi–vice principal Luna?!?" was all the student could articulate faced with such a demonstration.
"She prefer 'Nightmare Moon' under that form. Or just 'Nightmare' for short" told Celestia. "Back during our days as students, I was extremely popular, while Luna had been picked on for being 'geek'. She naturally became jealous of me and, like the fool I was, I didn't notice, let alone try to comfort her."
"From my jealousy grew hate, and the two combined gave birth to the form you see before you. Just like your own emotions gave birth to your own demonic form," added the Nightmare. "But don't go thinking that light show from princess Twilight and her friends killed your newfound demonic-side. All it did was free you of your negative feelings for the time being, causing you to return to your human form."
"Like me before in the past, you will have to learn how to not let your feelings take control of you, or you will be a danger to everyone. Maybe even more so, since your negative emotions were brutally purged from you, rather than dealt with in more conventional ways."
On cue, Celestia concluded "So from now on, and until we're sure you won't be a danger to others, you will train with Nightmare every night. Any question?"
The only one Sunset could think of, was how that change would affect her if she decided to go back to Equestria. But knowing there was no way the two sister knew, and not trusting her voice, she simply shook her head 'no'.
Nightmare was quite surprised the girl didn't ask about other people like them. After all, she might be the last student of CHS before Sunset to turn into a demon, the school wasn’t the only place in town, or even in the world, where peoples became demons. Oh, well… She will meet some eventually, anyway.
Sunset take lessons on how to control her demonic side?
Bonus: A yearly gathering of demons
Author's Note
Well, my story was so short it couldn’t be published, so here a little bonus. (Which somehow ended longer than the initial story. 
)
Hope you like it.
Bonus: A yearly gathering of demons
Luna, or rather Nightmare, had forced Sunset to come with her at the yearly demonic gathering, a sort of meet and greet for all demons in Canterlot City. Not that her mentor had any choice in the matter, apparently attendance was mandatory for all the local demons. As was the fact they had to come in demonic form, something crap about the need to preserve their human identity. (Not that it made much for her, since she hadn’t taken on a separate name when she first turned into a demon, back during the fall formal.)
Thankfully, the former-pony had made tremendous progress in her control of her demonic form. Not perfect yet, but all presents knew the up-hill battle it was to learn self-control in demonic-form, and most knew how to calm a fellow demon, so there shouldn’t be any problem.
To be honest, Sunset was quite impressed by how friendly and welcoming were her fellow demons. Weren’t they supposed to be manifestation of the worst in humanity? But again, if all were like her, peoples who simply were unfortunate to be affected by magic while having lost their way, but somehow came back from it, it wasn’t that surprising.
Naturally, in all community, even the nicest and most welcoming, there was a few jerks, as could confirm Sunset after meeting Chrysalis. While black, her skin seemed to shine, contrary to Nightmare’s who seemed to eat the light around her. She also had green insectile wings full of holes, as were her arms, legs and even hairs, which were just as green. From her forehead was a crooked but sharp horn that would have looked like a drill, if it wasn’t for the fact it was conical. She was wearing a ratty green dress that was just as full of hole as herself.
At first, Chrysalis had only noticed the Nightmare. "Well, well, well. If it isn’t the dumbass that revealed herself to some humans. Did you reveal your true self to anyone new, lately?"
Nightmare Moon let an exasperated sigh. "You know perfectly well, Chrysalis, that the only human aware of my current form is my sister" she said, gesturing to her body "and only because she was there when I first transformed, and was there to help me. So much she was exceptionally granted the right to keep the memories of the incident. Unless you decide to count our human side as different peoples, in case I suppose you can had young Sunset here, along your 'counterpart'."
"Ah, yes. Celestia being granted the right to keep her memories of us. The greatest mistake of the century, if you ask me!" condescendingly spat Chrysalis.
Then, pensively looking at the young demoness Nightmare had just pointed, she added "Sunset, Sunset… The name sound familiar… Ah! Isn't she the girl Celestia is trying to adopt? Say, dear aunt, is your sister aware her soon-to-be-daughter is a demon?"
"She does. The incident that made Sunset a demon happened at our school and involved a magical princess from another dimension. As a result at least some of the students are protected against having their memories of said event altered, but they seem to think her transformation was definitively reversed, not really knowing anything about how demons function."
Again Chrysalis looked at Sunset. "What do you say of ditching that spaz and her idiot sister and become my student instead? If you want I can even adopt you as my daughter."
"Not interested." was the monotone answer of Sunset.
"You'll regret it, Sunset…" said the insectoid demon, as it walked away.
Nightmare let out an exasperated sight as she put a comforting hand on Sunset's shoulder. "Be careful around Chrysalis, she's deceitful and dangerous. She's also a rare case in which the demon-side is an entirely different person from the human one. A shame, her human-side is a very nice person… You would love her."
A few weeks latter, the adoption of Sunset by principal Celestia was finally finished. Something she had always wanted from princess Celestia since said mare had taken her out of the orphanage, she finally got from her human counter-part. It made the former-pony happy.
It was also time for a greater family diner. On top of her mom and aunt, the two living together somehow, some distant cousin would visit to, a girl named Mi Amore Cadenza. Apparently, she usually came to those dinners with her fiancé, but the guy couldn't come this time.
As the door bell rang and Luna opened the door, Sunset saw a pale pink girl enter, with three-toned hairs of dark-pink, pale-yellow and purple. Once the girl had done her usual greetings to her 'aunts', she turned to her new cousins and asked "You must be Sunset?"
"Yes." Before the former-pony could say anything, she found herself hugged by the girl, who started crying, mumbling continuously something that sounded like 'sorry'.
Seeing how Sunset was befuddled, Luna explained "Cadance is the human-side of Chrysalis."