While the others gave Prince Mani a tour of the castle, Nightfall led Prince Helios to the library to show him the device.
"Alright, Twilight. Tell me how you managed to do this," he said.
Nightfall pointed to various parts of the device as he explained, "I used a thaumaturgometric rectified amplifier and focused it on the larger M-brane field, which I was able to connect with once I opened a channel leading outside of our universe. From there it was a simple matter of normalizing various frequencies..." Nightfall Twinkle's voice trailed off when he saw the look of surprise on Prince Helios's face.
"Twilight, do you realize the full gravity of what you've done here?" Helios asked. "You actually tapped into the larger M-brane field? Your device must have harnessed mana across all possible universes at the same time! My Sun, this type of transference, and the sheer amount of magic you must have funneled in order to pull your minds here without your bodies adjoined to them—you and your friends are incredibly lucky to be alive."
Nightfall's eyes widened as he silently did the math. "Oh my Stars, you're right!" he gasped. "I didn't realize what the mana surge could do because I forgot to consider the possibility of breaking the buffer! I mean, nopony has ever harnessed energy like this before... Oh, I'm such a foal! I could have killed myself and all of my friends. Prince Helios, this is the end of my multiverse experiments, I swear to you," Nightfall Twinkle said, lowering his head.
Helios took a deep breath, then spoke. "While I'm very relieved to hear that, I don't think you follow all of the implications. We can't risk sending your minds back the same way," he said. "We'd have a very good chance of ending up with twelve dead heroes, and I will not allow that possibility."
"No, I get that," said Nightfall Twinkle. "We just need to open a physical portal instead, and Nightfall's coordinates to our universe are still right here on the device. I can calculate how to create a specific portal back to our universe that will be safe. I could pull it off in a week, tops."
Helios's expression quickly changed to one that Nightfall had never before seen on Princess Celestia: a deep and vulnerable-looking sadness. "Twilight, this isn't like the alternate Equestria or Equestria High. As you've pointed out, your universe is perfectly symmetrical with ours," he explained. "Perfect symmetry. Do you not realize what this means? Perhaps it isn't registering because you don't want to accept the implications, but think very carefully. Our worlds are identical across the border, but any gate must appear as a mirror image..."
Nightfall Twinkle furrowed his brow in thought, and then his eyes widened. "But perfect symmetry for a physical portal would mean..." he said. "Oh no. Any attempt to walk through the portal..."
"...would be met by your double making the same attempt, barring your bodies from crossing, and thus your minds from switching places. Spells cast across the portal would automatically counter one another. Attempts at communication to negotiate a plan of agreement would be just as fruitless as they would be unnecessary," finished Prince Helios.
"But there are differences! Applejune is larger and heavier than Applejack, for one."
"Certainly. Which means if you opened a portal, you could end up breaking symmetry. But there's clearly a force opposing changes to symmetry, as you already noted, which is the only reason the symmetry exists. How strong must a force like that be?" asked Helios.
Nightfall paused to consider the idea. "You're not serious," he said.
"I fear that I am. Whatever force holds our universes in concert must be incredibly powerful to maintain perfect symmetry in everything else when sex and gender themselves are asymmetrical. Any attempt to break symmetry would be opposed by this force, and such an attempt would be guaranteed to fail by whichever means is most likely," said Prince Helios. "Opening a physical portal with the specific intent of creating a symmetry violation would spell immediate disaster, and it would probably seem coincidental. The symmetry force might even cause an earthquake large enough to level all of Equestria, but the mere destruction of your castle and everypony inside of it is a more likely outcome."
"Dammit, you're right," said Nightfall Twinkle. "Okay, so that's out. Then... then we have to do it the risky way."
"No, Twilight. Even if your friends agreed to risk death, it would not be acceptable to the future of Equestria—to either of them. Even if allowing that sort of suicide mission made ethical sense, you and the other Bearers are too important to sacrifice," said Helios.
"But, then you mean we have to stay here? Absolutely not! I won't allow my friends to suffer for this stupid mistake," said Nightfall, his eyes wet with emotion. "I know how much this would hurt them, and I won't let them make that sacrifice. Even death would be better."
"That is an exaggeration, Twilight. Try to remain objective," said Helios, in a gentle voice. "But you are correct that the situation is unacceptable, and fortunately, there is a third option."
"Anything," said Nightfall, wiping a tear from his cheek with a fetlock.
"If you and your friends are mentally connected to your doubles in the other universe, we can cast Skrifa Hugur. It would be difficult to invoke for this purpose, but if Mani and I assist you, it will work," Helios said, his voice calm and soothing.
Nightfall's stared into space, and then his face fell. "No, it wouldn't work. They would always know something is wrong, and once that seed of doubt exists, they could end up facing what I've faced all my life..."
"Then we won't tell them," said Prince Helios, his voice firm and direct. "They'll never know, and that's why the spell will work. There's a good chance they'll be fine."
"But we have to give them a choice, at the very least!" protested Nightfall.
Helios sighed. "For your friends to have a choice, they would need to know the truth. And if they know the truth, then what's the point of casting the spell? Do you really want your friends to suffer?"
"I can't accept this, Helios. There must be another way," said Nightfall, his eyes begging.
Helios gently shook his head. "I cannot think of one. The longer you and your friends remain in this universe, the more risk the spell will carry. I can give you an hour to think about it, and I will speak with Prince Mani to see if he has any other ideas. But after that, you must make your decision. Either we cast the spell that will return things to normal, or you and your friends must remain here forever and learn to deal with the consequences."
"They're still going to suffer. Physically, mentally..."
"There's a good chance they won't suffer at all. Certainly that's better than the alternative."
Nightfall shut his eyes tightly. "It doesn't matter. Both options could mean my friends suffering at my expense."
"You said you it was an accident to come here. Your friends will forgive you," said Prince Helios.
Nightfall's face was etched with lines of worry and despair. "Either we stay here forever, where they'll eventually forgive me, but certainly suffer; or we try a spell with painful consequences that might not work; and they might end up suffering anyway. And if they learn how the spell works, they might not forgive me at all. I still think attempting the transfer a second time is a better option."
"Really? You would rather risk their deaths than cause them a minor inconvenience?" asked Helios.
"This is not a 'minor inconvenience'. Of all ponies, I should know. That spell is not an option, and neither is staying here," said Nightfall, his voice quivering with anger.
Prince Helios frowned. "Alright then. Let's put our entire hoof on the table, shall we?" he said. "You're willing to risk your life, and the lives of your friends, to avoid feeling a tiny bit of guilt for you-know-exactly-why. Is that it, then? Assuaging a princess's ego is more important than the lives of your friends?"
Nightfall Twinkle pretended not to listen. "You no longer outrank me, Prince Helios. You can't force me to do this to my friends. If we made it here in one piece, we can make it back the same way."
"You don't know that, Twilight!" growled Helios. "And whether or not I outrank you, I will destroy this device rather than allow you to bring about the death of your friends. Fortunately, I don't think it will come to that. Even in this state, you are not so blindly selfish."
Nightfall Twinkle covered his face with a hoof, near tears. "But it's all my fault! You want my friends to suffer because of a stupid mistake I made, while I... I feel, just, unbelievably guilty," said Nightfall Twinkle. "I never wanted this to happen."
"I know," said Prince Helios, placing a hoof on Nightfall's shoulder. "Welcome to royalty, son. It's a life of hard choices."
"Can't we at least let my friends in on the risks involved? Why do I need to make the call?" asked Nightfall.
"Twilight, we've already gone over this. The spell won't work if we communicate any details in advance," Helios said. "That would remove the option entirely."
Nightfall closed his eyes for a few moments, the wheels turning in his head. "There really are no other options... It's so unfair," he finally said.
"I know," said Helios. "It truly is unfortunate, but sometimes that is how life works."
"Is this my destiny, Prince Helios? Is this what I've earned for being crazy enough to want this?" Nightfall asked, pointing at the stone archway. "Here I am in a castle, surrounded by friends, and I'm still pushed by this desire to make myself into something more than I am, to be admired by ponies, even to have control over Nature herself. I mean, this need I've had all my life is what motivates me to excel, but it keeps pushing me over the edge. I keep flying too close to your Sun, and until now I'd never been burned," he admitted. "I just can't believe I actually did it this time. My poor friends."
Prince Helios gave a wan smile. "Normal ponies aren't the ones written about in history books. It is desire, and perhaps even insanity, that makes outliers like you and I into the remarkable ponies we are. I suppose in a way, yes, this is your fault. But it is not in the way you are thinking," he said. "This is not karma or payback for anything you have sought to achieve. It simply is what it is. You know, I once told a young pony named Nightfall Twinkle that being a prince requires incredible sacrifice. Including sacrifices from one's friends: even if they never signed up for it, and even when they don't deserve it. I'm sure Princess Celestia has told you the same. Be happy for your fortune, Twilight Sparkle, and be happy for the fortunes of your friends. Remember why you studied magic in the first place. Your friends would sacrifice everything for you to be happy. They aren't suffering because of you, but because they love you. And that love, I believe you have earned."
Nightfall Twinkle nodded, and Prince Helios reached out to wipe another tear from his cheek.
"I don't want to hurt my friends this badly, and I don't want to lie to them..." Nightfall said, gritting his teeth.
"You already know the choice I would make, Twilight, and I hope you will heed my counsel. However, as you have pointed out, I can't make that decision for you. It is yours alone to make. But neither choice is without dire consequence," cautioned Helios.
Nightfall nodded, sat quietly on the floor of the library, and brooded.