Ok, something I'm playing with, and dunno ifn I'll finish it or not, but figured I'd throw it out here for y'all. Working title is The Third Choice. Mostly inspired by the latch key kid revelation, it is inspired by a number of stories I've read.

Twilight looked around, startled as the whole castle shivered. She knew it wasn't alive, at least not in the sense that ponies were, but as the Princess of Magic, she could feel an ebb and flow much like a pulse running through it. Calling out to Starlight through magic as much as voice, she asked her if she felt that, receiving an affirmative reply from her fellow princess.

Starlight trotted into the room, joining Twilight and Spike, “What do you think it was?”

She shook her head, “No idea, but everything feels like it’s back to normal.”

Nodding her head and smiling, Starlight grinned, “Another day in Equestria? At least we don’t have to go and save the world again, unless you missed something? Speaking of days, we’ve been so busy, what is today?”

Spike spoke up, taking this one, telling them the date, “And I know this because someone has to keep track of it with how often you two forget to come up for air.”

Instead of the exaggerated pout the two princesses were apt to give him for that, Starlight got a shocked look on her face, “Twilight, I need to do something, and you need to gather the other princesses. I’ll be back in two days.”

Galloping out the door and launching herself into the air, Twilight barely got to start asking a question before Starlight was gone.

Cadance, Luna, Twilight and Celestia sat chatting casually in the throne room, the question of why they were here already answered so much as it could when Starlight entered.

Bowing her head, “Your highnesses,” only to be cut off by Celestia.

“Starlight, you know you don’t have to address us like that any more, not since you joined us.”

Shaking her head, “I’m afraid I do, for this matter I can not be a princess, but only a pony.” Taking a deep breath, “After being injured by Twilight’s surge during her exam, I took a week’s vacation. During that trip, Star Note, the mare who all but raised me, told me a secret, but swore me to keep it until a specific date. Two days ago, that date passed, the day the castle shuddered.”

Stepping to the doorway, she spoke to someone just out of sight before returning to the room, followed by a unicorn mare with deep violet fur and a pale blue mane. Luna’s horn lit suddenly, quickly followed by her sisters. Cadance looked confused, while Twilight’s head tilted to the side as she looked at their guest in curiousity. Starlight moved to put up a shield only to be stopped by Star, “Don’t. I do hope they’ll hear me out, that they’ll trust that I’m not a changeling, but if attacking me is what they desire, then they may, I certainly deserve it for what I did.”

It was Twilight who spoke first, “What did you do? If everything Starlight has told me is true, you’ve been a wonderful friend to her, a teacher, and a mentor. If not for you, she wouldn’t be the pony she is today.”

Nodding her head, the unicorn licked her lips nervously, “I suppose that is all true, because if she hadn’t had me there,” she let the illusion spell she had worn for almost fifteen years fall, “she would have wound up as me.” Standing before the shocked princesses was Starlight Glimmer. Starlight Glimmer the unicorn, not the alicorn princess. A Starlight Glimmer that looked to be in her nearly forty.

Taking a deep breath and standing tall, “My name is Starlight Glimmer, and I stand before you to be judged for my crimes. I have committed high treason, broken into a number of Royal facilities, studied and wielded forbidden magic, attempted the murder of Princess Twilight Sparkle, attacked one of the Element Bearers and doomed Equestria to war several times over, and finally, destroyed Equestria.”

Love the idea of Alicorn Starlight. I have to ask though, which one is our Starlight and what the heck is going on?

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Grins, "I suppose it's the Celestia in me, but I hate to make it that easy, and prefer that you guess. That said though, I will point out a few clues in there, specifically the working title, and the age of canon Starlight (the older unicorn), and I will be happy to answer once you've taken a shot."

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Okay.