After Twilight Sparkle had agreed to replace Celestia and Luna as Equestria's singular ruler, the former had taken her aside so they could talk about something in private, as if she weren't having a moment with a paper bag. It all felt kind of surreal, how the princesses had suddenly sprung their desire to retire on her and her friends, with a ceremony planned to be held in a few days no less, and had pressed forward with it — along with some input from her friends — until she had caved.
Eventually, though, the quiet, sedate walk was enough to calm her down. Since there didn't seem to be anyone else around in the corridor that they were passing through, she decided to try and sate her curiosity. "So, uh... What exactly did you want to tell me?"
"I'll tell you once we're in a more secure location," Celestia assured her.
That location turned out to be the princess' room. Except it wasn't: because, as soon as the door was closed behind them, Celestia's horn lit up, and that was Twilight Sparkle's only warning before they were both teleported to the space-like place where she had become an alicorn.
Celestia turned to her and said, "Now we can speak candidly."
"Okay..." Was Twilight Sparkle's uneasy response, wondering what more was in store for her after being brought to such a place.
"What I'm about to tell you is so secret that only I know it," Celestia solemnly began. "And knowing it will come with an additional responsibility, which is partly why I brought you here."
Gulping, Twilight Sparkle asked, "What is it?"
"Aliencorns exist," Celestia stated without batting an eye.
Twilight Sparkle's eyes shrunk into pinpricks upon hearing that. She had strongly rejected the existence of aliencorns in the past, seeing them as food for conspiracy theorists, or fantastical fictions to get attention and make money from the easily-fooled. But if the princess was saying that they were real...
"They're actually real?" She muttered her internal struggle aloud.
"Technically," Celestia corrected. "When I say that they exist..." She cast a spell, and in her place now stood an aliencorn, via an illusion. It had no mane or tail, its skin was grey and fully exposed because of a lack of hair, its body was sexless and unnaturally thin, and its eyes were large, bulbous and black. "...This is what I mean."
With mouth hanging open and eyes at their widest, Twilight Sparkle was slow to sputter, "Y-you are the a-aliencorn?" Realizing how absurd that sounded, she reached the conclusion that the princess must be pulling a prank on her, so she laughed nervously and awkwardly said, "That's a good one... Very funny..."
"This is no joke," Celestia replied, using an echo-y, oscillating voice. "You will need to assume the guise of an aliencorn for the sake of Equestria's future."
"I don't understand," Twilight Sparkle replied with a shake of her head, who was immensely confused. "How is pretending to be an aliencorn supposed to be good for Equestria?"
Celestia dispelled the illusion before motioning to their environment while she glanced around at it herself. "This is the Herd Dimension, where ponies transcend to and become alicorns after they perform a good deed that helps many others selflessly." Her gaze turned away from their surroundings and locked with Twilight Sparkle's. "However, the qualifications are relatively low. In the distant past, it was sufficient enough for alicorns to be extremely rare, particularly during the period when the tribes segregated and fought among themselves, but these days there would be hundreds of alicorns without intervention."
That was a lot to take in and digest, so it took Twilight Sparkle a moment to decide on how she would respond to it, under the assumption that what Celestia was intimating at the end was true and well-meaning. "So, too many alicorns is a bad thing... And the aliencorn plays a role in preventing ponies from becoming alicorns somehow?"
After nodding her head, Celestia explained, "Not controlling the number of alicorns is bad for two reasons. The first is that there is no requirement for a pony to be wholly good, and even bad ponies can do good, selfless things given the right circumstances. If ponies are not first taught and tested, like was done with you, there's no telling how many Nightmare Moon-like alicorns we would be faced with."
"The second reason," she continued, "has to do with the power structure of the country. When the tribes were united and Equestria established, it was decided that alicorns, representing all three tribes, should be given the responsibility of ruling, with the hope that it would avoid any discrimination that was in favor of, or against, any particular tribe. While it had been odd to have two rulers at the beginning, it wasn't long before I was ruling alone, and that remained the case for a thousand years."
"...And that's why alicorns are granted royalty status and expected to rule," Twilight Sparkle deduced. "But since there's only one true seat of power..."
Celestia smiled. "Correct. Cadence wasn't an issue because her fate lay with ruling the Crystal Empire, and you will replace Luna and myself. If there are any more alicorns, however, there is no place for them to fulfill the role of a ruler, with the potential exception of being a co-ruler with you."
Brow furrowed in thought, Twilight Sparkle asked, "And how does the aliencorn factor into this?"
"Well," Celestia began, who once again turned her attention to their surroundings, "that's because I had to master the Herd Dimension, in order to control the creation of alicorns." She returned her attention to Twilight Sparkle. "However, because one has to be in a transcended state to be here, they won't forget their experience, nor can it be altered, buried or erased. So, as you can imagine, it wasn't feasible to keep sending ponies back with the knowledge that I denied them alicornhood, regardless of the reason."
"So, a long time ago, I conceived the aliencorn to hide my identity and motive," she said, getting into the meat of her answer. "And, as you know, my strength isn't in acting or stagecraft." She cleared her throat. "Fortunately, I didn't have to use the aliencorn often. It didn't become a problem until the Rose Well incident, between the amount of ponies who witnessed Brazen's disappearance and reappearance, and how much publicity the story got because of how far we've advanced technology-wise."
Twilight Sparkle couldn't help expressing her long-standing disbelief. "So that was true all along? It really was a cover-up?"
Celestia nodded her head. "After that, I recognized not only the problem I was faced with at the time, but the challenge I would have in the future, between the growth of the population, the advancement of technology and magic, and the goodwill that I continued to engender. I had to expend some of my magic to seal the Herd Dimension away from other ponies entering, until I was ready to receive them, and if there was more than one pony I would place a seal on all but one to control the flow."
"Wait," Twilight Sparkle interrupted, as the pieces were being put together in her mind. "So when I completed Star Swirl's spell, the seal on me was released?"
"That's right," Celestia readily admitted. "You and the other element bearers were all sealed the day before I sent you to Ponyville."
"Now," she picked up where she left off, "since Rose Well I've waited until ponies are asleep and use illusions to make it seem like they're in a dream, because all who enter the Herd Dimension are fully cognizant. They're not actually dreaming, of course, so my success is rather... mixed."
Twilight Sparkle quickly prompted, "And why continue to disguise yourself as an aliencorn?"
Celestia looked askance and blushed. "I got a lot of practice with it, so I tend to fall back on it when another role I play within the fake dream doesn't... work out." She regained her composure and added, "That aside, from time to time I use the aliencorn to keep the idea alive, since it misdirects from the truth."
"What about the probes, then?" Twilight Sparkle pressed.
Celestia turned her head away and started to tap the points of her forehooves together without falling forward. "Would you believe me if I told you they were complete accidents?"
Twilight Sparkle pressed a hoof between her closed eyes and released an exasperated sigh. "There's no way this can be real."
"Whatever do you mean, Twilight?" Celestia innocently asked. "Don't you believe me?"
"Now, what did that book say to do, when you want to wake up from a dream?" Twilight Sparkle wondered aloud. A second later her face lit up. "Oh, yeah!"
She forcefully applied a hoof to the side of her head, and the next thing she knew she was opening her eyes and looking up at her canopy, which she could barely make out in the dark.
"Ugh!" Twilight Sparkle complained internally as she fitfully rolled onto her side and adjusted her covers. "I can't believe my subconscious mind would try to rationalize all of those absurd things..." She paused when she noticed something odd. "And why I am sore there?"
Had she stood near one of her bedroom windows at that precise moment, she would have seen a flying saucer moving away from her castle and quickly making its way to space, its inhabitants happy that they had distracted her long enough to complete their probe.