The Whole Thing
"Princess Luna, Princess Celestia, I need to talk to the both of you," Twilight said as she walked into the personal chambers of the princesses. She sat down with a humph as she looked up at her mentor, disappointment and anger in her eyes.
"What is it, Twilight?" Princess Celestia asked.
"I have been doing a lot of thinking these past few months and have come to a startling and disturbing conclusion."
"That is?"
"After the invasion of the Changelings and your subsequent defeat at the hooves of Queen Chrysalis, things just didn't add up. Even if Chrysalis was fully charged with the power of love from my brother, how is it possible for you to be overpowered by her? You raise the sun!" Twilight was pacing backwards and forwards now, "Not only that, Chrysalis also failed to kill you, a clear oversight on her part despite being able to beat you in a battle of raw magical power, the question remains: why did she do so? Everypony knows that you were the only one capable of defeating her if her powers faded even slightly. So why take the risk? It made no sense!"
The princesses exchanged looks.
"Even if we take into consideration that you were really overpowered by the changeling queen, and even if your powers were not fully charged because of some reason, and taking the lack of concern you displayed during your so-called capture, I have come to a worrying and disturbing conclusion." Twilight stopped and looked at the two of them panting quickly, "One that would explain why all this was possible, and also why Princess Luna was nowhere to be seen during the entire invasion."
"And what is that?" Celestia asked calmly.
Twilight took a deep breath and looked her mentor in the eye, "It was all an elaborate setup. A lie. The invasion was fake and was an inside job fully planned out and executed by none other than you!"
Celestia blinked at that. "What?"
"Yes, you heard me! I'm accusing you for it. I don't know why, but it's the only thing that makes sense! The reason why you were defeated was because you let yourself lose. The reason why you weren't worried about the Queen or her soldiers harming anypony was because you knew that they wouldn't. How can there be a hostile takeover with no casualties? It makes no sense! Why didn't I see it before?"
The princesses stared at Twilight shocked.
"How Chrysalis let me find Princess Cadence in those caves! How she let me bring her back, using those mares to guard the exit instead of her changeling soldiers? How Princess Cadence was allowed to stay near Shining Armor and restore his memory and power through her own magic when Chrysalis spent so long in keeping them apart! It's all so obvious, why didn't I see it before..."
"You're rambling, my dear," Celestia informed her.
Twilight stopped and lowered her eyes to the ground, "The Queen of the changelings... was... Princess Luna!" She pointed an accusatory hoof at the Princess of the Night.
There was a long silence.
"Please tell me I'm wrong?" Twilight asked looking up. When silence met her question she pressed the point, "I'm wrong, right?"
Luna stood up and walked over to the mare. "No."
Twilight couldn't believe her ears. "Y-y-you were?"
"I was. Am."
Shutting her eyes tightly closed to absorb the shock of what she had just heard, Twilight turned on the princess. She glared up at her, anger evident in her eyes, "But... but why?"
Luna smiled at Twilight gently, "I am impressed that you have made such a discovery. I had thought that our plan to help the Princess of the Crystal Empire had worked without arousing any suspicion. She truly is a bright student, one that you have underestimated, sister."
The Sun Princess shook her head, "No. I expected that one day she might discover it."
"Oh? Well, I shall answer your question, Twilight Sparkle. Princess Cadence is the princess of the Crystal Empire. Her position in our kingdom was nothing more than a figurehead of a fabled kingdom that had vanished without a trace over a thousand years ago. When the Crystal Empire disappeared because of King Sombra's curse after we had defeated him, Cadence's ancestors were forced to live in Equestria as exiles."
"Cadence isn't... immortal?" Twilight asked.
"She is as immortal as any alicorn can be, but her line is that of unicorn. She is... special."
Princess Celestia smiled at Twilight, "Just like my faithful student."
At those words, and despite her earlier anger, Twilight couldn't help flush at the praise from her mentor. She looked at her wings and folded them backwards, "But... why did you have to go that far?"
Luna once again started to walk around Twilight, talking to her as if she were a student in her class. Twilight listened to every word as she spoke.
"As she is the Crystal Princess, we needed to establish her as a powerful and capable leader able to protect her ponies from harm. You have to understand that prior to the reappearance of the Crystal Empire, your sister-in-law was seen as something of a..."
"Wastrel."
"Sister!" Luna snapped at Celestia. "You need not be so blunt."
"It is time she learns the ways of the courts, Luna. It isn't always pleasant, and not everypony sees reason. Greed and power are at constant play, and weakness is shunned. Displaying Cadence as an assertive ruler was a necessity that we had to... orchestrate."
"Then... is that why you allowed yourself to be beaten by Chrysalis- I mean Luna?"
"In part, yes. If we had allowed Cadence to rule as she was, nopony in our kingdom or our neighbors would have taken her seriously. She would command no respect and will forever be dependent on us to rule over her realm. Celestia and I agreed that it was necessary for this charade to take place to acclimate our subjects to a new and powerful alicorn."
"But it's a lie! You lied to us all!" Twilight shouted, "Lied to me."
Princess Celestia looked at her faithful student with compassion, "Yes. We had to lie to you all. You especially, my faithful student."
"Then... even that time was a test? For me?"
"You saw through our ruse even when we had tried to postpone it for as long as possible. You had found Princess Cadence at the right moment and revealed her perfectly in front of all of Canterlot to see. If you had been informed that it was all just an act, then you wouldn't have fought as hard as you did and there would have been those that would have suspected foul play. Until now, you have been the only one to actually solve the puzzle."
"I nearly died!"
"You were never really in any danger, Twilight. Never, at any point."
"The caves? You knew?"
"I've been alive for over a thousand years, Twilight, and a vast majority of that time I've lived here. Do you really believe I would have no idea about the caverns beneath my own home? There is no reason for any of my ponies to go down there, so I used it to keep Cadence safely hidden away until it was time for her big reveal."
"But... but Chrysalis' magic was green, and Princess Luna's is blue. You can't disguise your magic... can you?" Twilight still refused to believe that it was all an act.
"I had given Luna some of my power to maintain her changeling form. I had also given some of my power to many of our most loyal Guards to maintain their changeling appearance. The mixture of our powers together... well, what do you get when you mix blue and yellow?"
"Green. That makes sense," Twilight said to herself. "You thought of everything."
"And you still saw right through it once you applied what you know to what had happened, as is expected from you, Princess Twilight."
Again, Twilight's face blushed again at the praise.
"So, let me get this straight. You orchestrated the entire invasion of Canterlot to promote Cadence's appearance to your subjects-"
"Our subjects, Twilight," Princess Celestia reminded her student.
"Our subjects. You did all that just to prove that Cadence is a strong, capable ruler so that she will be respected and admired as the leader of the Crystal Empire, which was still missing at the time of the invasion, and this was done by concocting strange creatures called changelings who followed their queen, which you have admitted were actually guards that you had disguised probably through the use of some transfiguration magic so they could have just turned the incantation off and look like their normal selves-"
"It's a divergence of the spell they use to keep their coats and their eyes uniform," Celestia pointed out. "A type of vanity spell. Surely you don't think that I'd only keep white pegasi and unicorns in my service, do you? Or that Luna only keeps those with dark blue or indigo coats."
"No, I guess not... but don't try and change the subject," Twilight replied. "How did Chrysalis -- I mean Luna -- and the changeling army get blasted away by the spell?"
"Although I had expected her to use some sort of repelling spell to banish us from Canterlot, I had severely underestimated the strength of the spell. It took me a while to recover and teleport back. Luckily I managed to do so in time for the reception. I am still very upset that I missed my own niece's wedding, but at least I can say that I helped her cement her position as the Princess of the Crystal Empire."
Twilight blinked for a few minutes, "Are the Canterlot courts really difficult to get along with?"
"You have no idea," both the princesses stated together.
"I had hoped that you would understand why we had to try and do this, Twilight. Sometimes I forget that you haven't lived and learned for as long as we have. There are some things you are going to find out about our kind that will put to the test everything you've ever believed. We do not like manipulating our subjects this way, but sometimes it is a necessity in order to maintain harmony."
"Will... will I have to do this also?"
"One day, yes."
"Then... was putting Cadence as my sitter also part of the plan to help shape me?" Twilight asked, afraid of the answer.
At this Celestia shook her head, "Actually, she was taken by you. I had nothing to do with that. You did. Then, afterwards, she fell for your brother. If it weren't for her, I would have never found you, Twilight."
"I'm sorry, Princess. This is too much for me to handle right now. I'm going to have to think for a bit. I'll be okay."
"Take all the time you need. We'll be here. I only ask that you do not endanger your sister-in-law's life by revealing what you have learned here to others."
"And believe us when we say that there will be those that will try to take your sister's throne by force if they sense any kind of weakness," Luna added. "Your brother won't be able to protect her all the time."
"Don't worry. I won't tell anypony."
With that, Twilight left.
As soon as she was gone, Luna turned to her, "Why did you lie and say that you didn't purposely send Cadence to foalsit her?"
"I didn't. I sent Cadence there to deliver a letter for her father. She fell for their son after she felt his love for her, you know. It was through that relationship that she ended up foalsitting Twilight Sparkle as an excuse to be with him."
Luna looked at Celestia for a moment before shaking her head, "You are so full of hot air sometimes, sister."
"I'm serious!" Celestia tried to convince Luna, but failed miserably.