Magic Is A Pain Sometimes

Sunset felt hot. It was only May, yet it felt like the middle of July. Then she felt herself being surrounded by magic and knew she had Ponied-Up.
"Sunset Shimmer, can you please try to control your magic?" barked Principal Celestia.
"Sorry, miss, I really didn't mean to. I'll try not to let it happen again," she replied.

The problem was, Sunset did not know how to. This was a recent problem. She would suddenly grow a magical pair of ears and a tail, despite there being no threat to the school. It was becoming difficult to hide her magic from prying eyes in public, which meant that there was a risk of people outside of Canterlot High School discovering Equestria and exploiting magic. Sunset herself knew how that ended; a demon threatening to tear the world apart with uncontrollable magic. Due to bullying, it had happened very recently to Pinkie Pie and her boyfriend Cloudy Sky, but it had all been sorted out. Yet that did not answer Sunset's question about her spontaneous magical powers. There was only one person, or, really, pony, who could help. Princess Twilight Sparkle of Friendship in Equestria.

After school, Sunset took the bus home to her tiny bungalow. She unlocked the door, walked inside, closed the door with magic and made her way to the kitchen table where her journal was waiting. She opened it and grabbed a pencil. What to write? There was so little to tell her pony friend about. Twilight already knew about Pinkie and Cloudy's ordeal. What else was there to say, other than her random bursts of magic? She sighed, and wrote about the first ideas that popped into her head.

Dear Twilight Sparkle,

I know I only wrote to you recently, but I feel that you need to know about the events of the last two weeks.

I have encountered a problem (once again). I have started to Pony-Up at random, although I am not using any magic. I'm worried that CHS is once again under threat, even though there is nothing obvious to worry about. As you can probably guess, it's not easy to explain. But I know as a friend that you understand. Why do you think this is happening to me? Is anyone in danger, or am I just panicking because of the excess magic running through my system?

I hope that this is a simple illness and that I will stop growing ears and a tail in public as soon as I recover. Please can you contact me as soon as you have an answer.
Your pen pal from another universe,
Sunset Shimmer.

The reply was instantaneous and did nothing to soothe her nerves. Oh my goodness Sunset. CHS is in BIG trouble! Stay alert. Tell me if you need me to jump through the mirror. I'm hyperventilating as I write. You're all in big BIG danger. That doesn't make sense. Never mind. Please try not to get killed. We've had too many close calls. I sooooo hope that this isn't huge. But I think it is. Bigger than anything you've faced so far. Stay safe. I really have to go. Twilight Sparkle. It just confirmed everything that Sunset was dreading. She couldn't deal with another demon running around.
Of course you can't deal with any more demons, you weakling, said a voice inside her head. Her own inner demon reminding her of her past.
"Shut up, you, I never asked for your opinion," Sunset told it. "Anyway, I thought I'd banished you."
I couldn't disappear just like that, could I? You're amazing company! I enjoy being with you.
"I enjoy being with you when you don't insult me. Please. Make yourself useful and think about why I'm turning into a pony every five minutes."

Sunset never questioned why she spoke out loud to the voice in her head. Why she never questioned her sanity. She didn't know if any of her other friends who had become demons - the human Twilight, Pinkie, Cloudy and Gloriosa Daisy from Camp Everfree - still had their demons lingering. She knew that Twilight had experienced problems with Midnight Sparkle, but whether Midnight was still in existence or not, Sunset didn't know.

Eureka! I have it! There must be a hidden magical threat, possibly a portal, somewhere in the area. Other than the statue of the horse in front of Canterlot, I mean. There are two possibilities: either a new threat, or an old one. You have any enemies? Wrong question. What I mean is, do you have any enemies who really hate you and would willingly use Equestrian magic to eradicate you and your friends? Am I a friend? Am I going to be destroyed in the same way that I almost destroyed both this one and Equestria?
"Woah, too many questions! Let's stick with the most important thing: who would use Equestrian magic to wipe me out of existence. I have a lot of people and ponies who dislike me, but nobody I can think of would try to kill me or torture me. I think so. I mean, I don't have a clean record, but I wasn't that bad, except when you showed up. No offense, but you caused a lot of trouble."
None taken. Plus, I might have an answer. Someone after me. Can't remember who. Girls, I think.
"There's been loads of girls."
Three girls together. After me but before Midnight Sparkle showed up. Lots of singing.
"The Sirens!" shouted Sunset. "I still don't know what to make of you, but you're good when you put your mind to something. Ok, our mind. You get the idea. The problem is, I now have more questions. What did they do and where did they go after my friends and I defeated them? They couldn't sing any more because their crystal pendants were destroyed. Have they found new ones? Oh no. Back at Everfree, the crystals in the cave were magical... They probably have." It dawned on Sunset and her demon that they had released magic into the world and could have caused even more trouble than they bargained for.