Celestia awoke with a pounding headache and the feeling that she had just flown halfway across Equestria. "Ugh," she murmured as she blinked her bleary eyes. "What happened?"
"Take it slow, little one," a deep voice told her. "I think you overdid it just a bit."
She shook her head and peered up into the dragon's concerned filled face. "Are you ok?" she asked as she tried to scramble to her feet.
"I'm a bit worse for the wear, but all things considered I'm doing great," he chuckled as he gestured towards his battered but now free hindquarters.
"Amazing! It actually worked?" Celestia exclaimed as she unsteadily rose to examine him. "I mean, I hoped it would, but it was pretty close, wasn't it?"
The dragon nodded as he glanced back at the heavily scraped scales on his back. "We just barely managed it," he agreed. "But wow, your power is pretty amazing. I don't think any other ten unicorns could have done something like that."
"It's not like that. I'm sure my mother could have done it without even breaking a sweat," Celestia replied with a slight blush. "She's a lot stronger than I am."
"If that's true, then ponies sure have changes since I've been down here," the dragon laughed. He gave her a broad smile as he carefully stood up, mindful of the low ceiling. "I don't know how I can ever repay you for this incredible deed."
"Repay me?" she asked. "No, there's no need for that. I just did what anypony would have done."
"Perhaps, perhaps not," he replied. His eyes widened. "Perhaps I can help you with this Tirek fellow!"
Celestia frowned. "I don't think that would be a good idea," she pointed out. "You've been trapped down here for so long, surely you just need to take it easy for a while."
"Nonsense," he retorted. "I've never felt better."
"Your knees are wobbling," she pointed out.
He snuck a guilty glance back at his clearly unstable hindquarters. "So they are," he chuckled. "I'm sure it will pass momentarily."
"There's no sense in getting you loose just for you to run off and get hurt," she insisted. "I wouldn't be able to forgive myself if something happened to you because of me."
"But-"
"No buts!" Celestia scolded. "I won't hear another word."
The dragon let out a great sigh. "Fine," he relented. "But I will not forget this debt."
"You should just go see if you can talk to your friends," Celestia retorted. "Just be really careful. I might not be able to dig you out next time."
"Will you be continuing on your quest for the Rainbow now?"
"Yes, I'd better," Celestia reluctantly answered. "I need to find my little sister as well."
"Your little sister is here? Well, it's not much, but I'll certainly keep an eye out for her," the dragon promised.
"Thank you." Celestia said as she walked towards the exit to the room. "Hopefully I'll see you again on my way out!"
"I hope so!" the dragon called as she left the chamber. "Come to think of it, I didn't even get her name," he muttered regretfully.
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Luna yawned, stretched her tired legs, and reluctantly stood back up. "I wonder what time it is," she murmured sleepily as she resumed walking. "It feels like I've been going for hours!" She glanced around, wondering if anyone was silently watching her. "Hello?" she called out, but there was no answer. "They never said I'd have to go the whole way all by myself."
She was glad that there hadn't been any other hallways along the way, else she would seriously be wondering if she was lost by now. She had passed many doors on her way, but all the ones she'd tried had been locked. Thus this had to be the right way. It was almost as if the castle itself was guiding her now.
"I hope I find Big Sister soon," Luna whispered. "I want to go home."
It was only a few minutes later that she first spied a faint light shining from far down the hallway. While her entire path thus far had been dimly lit, this light was markedly different, almost as if daylight waited for her on the other side. She increased her speed to a gallop, eager to reach whatever it was.
Luna skidded to a stop as she entered a large brilliantly lit chamber. The light seemed to be coming from something hovering just above a lavish marble pedestal in the center of the room. As her eyes adjusted to the increased brightness, she could barely make out that it was a heart shaped pendant.
"The Rainbow!" she exclaimed.
"Luna!" a very familiar voice called out from the far side of the room.
"Big Sister!" Luna yelled back. "We found it!"
Celestia broke out in a wide grin as she dashed towards her sister. "Never mind that, you disobedient filly! You shouldn't have gone off on your own. I was so worried," she scolded.
"HALT!" an overwhelmingly loud voice commanded.
The two ponies froze in place. "Wha-what was that?" Luna stammered.
A shimmering apparition in the rough shape of a pony suddenly appeared between the two. "What business do you have with the Rainbow of Light?" the ghostly creature demanded.
"We need the Rainbow's power to stop a terrible monster," Celestia answered as confidently as she could manage. "I promise that we'll bring it back just as soon as we can."
"Silence!" the spirit replied. "Such impertinence! You would dare take away the only thing that keeps this castle alive?"
"Well, we..." Celestia hung her head. "I guess Buttons's theory was correct then."
"What do you mean?" Luna ventured to ask.
"If we take the Rainbow, then all of the ponies here will just fade away, right?"
"That is correct," the spirit confirmed. "Its power is the only thing that still binds us to this plane."
"I... I understand," Celestia replied. "If that is truly the case, then I apologize for bothering you." She glanced at Luna. "Come on, we should go."
"But what about Mama?" Luna asked as she trotted over to her sister's side. "You said she wouldn't be able to stop the monster without the Rainbow."
"I'm sure she'll manage somehow," Celestia assured her. "After all, there will be a lot of other ponies helping her this time."
"But we came so far!" Luna whined. "And I went through these hard tests and everything."
"If we take the Rainbow with us, Buttons, Fizzy, and every other pony in the castle will be gone forever," Celestia retorted. "There's no help for it. We'll just have to find another way."
Luna reluctantly nodded. "They were all really nice to me," she admitted. "Especially Buttons and Truly."
"Wait!" The apparition cocked her head to the side. "Is the situation you face really that serious?" she demanded.
"It is quite dire," Celestia admitted reluctantly. "But I could never-"
"There may yet be a way," the spirit interjected. "But it would demand a... sacrifice."
"What do you mean?" Celestia asked fearfully.
"I could allow you to borrow the Rainbow for a short time... if a sufficient source of power were to take its place." The ghostly head nodded towards the smaller pony. "I sense great power in both of you girls. I think the baby pony would do quite nicely. Quite nicely indeed."
"What are you saying?"
"It's quite simple. You take the Rainbow, and we keep your sister," the spirit replied.
"Absolutely not."
"I assure you, your sister would come to no harm," the spirit promised. "However she would never be able to return to your world. She would remain here as one of us. Forever young and safe, even long after the world above changes once again."
Celestia tossed her head angrily. "What are you saying?" she demanded. "There's no way I'd do that to Luna."
"But if it would save Mama," Luna said nervously.
"Not a chance."
"Perhaps you do not understand the true extent of the Rainbow's power," the spirit chuckled. "Defeating monsters is but a trifle. With its power you would be the mightiest pony in the world. Anything you desired would be yours; you would only have to reach out and grasp it."
"That is not the sort of power I would wish for," Celestia replied. "Nor would such a power ever equal the value of my sister to me." She turned to leave. "Good day."
"Such stubborn loyalty," the spirit sighed. "It truly is refreshing."
Luna's eyes widened. "Wait, Big Sister," she cautioned. "Look!"
The spirit's body rapidly grew more and more opaque until a quite normal looking light-pink unicorn stood before them. She gave the two surprised fillies a warm smile as she tossed her long purple-streaked white mane out of her eyes. "Hello there, my name is Twilight," she introduced herself. "Please forgive me for making you two undergo all of these bothersome trials, but the Rainbow is truly too powerful to allow any but the purest of heart to wield."
"You're the pony that enchanted this whole castle?" Celestia asked.
"Yes, with the help of my dear friend, the Moochick," Twilight confirmed. "Sadly he is no longer with us."
Celestia looked wary. "So was this whole fading away thing just another trial?"
"Oh no," Twilight answered. "The magic that sustains us will fail once the Rainbow is taken away." She stepped up close to Celestia and laid her hoof on the filly's shoulder. "However that fate is inevitable. The spells have been steadily weakening over the centuries. They will not last too much longer regardless of the Rainbow's presence."
"Can't the spells be recast?"
"If I were yet in my prime, and the Moochick was still here, then perhaps." Twilight shook her head. "However that is not the case. That is why I want you to take the Rainbow with you now."
"I already told you I can't do that," Celestia protested. "Even if it can only give you all a little more time, it still wouldn't be right."
"When the enchantments fail, the castle will likely collapse, and the Rainbow of Light will be buried forever. That is not what any of us would want. Not when the Rainbow can still be used for so much good in this world."
"So Buttons and Truly and Lickety Split and Baby Moondancer are all going to die?" Luna asked, her lip quivering.
"In a way we all died a long time ago," Twilight answered quietly. "The spells were only meant to temporarily preserve us until we could be rescued. Unfortunately the Smooze was far too thick, and far too much time passed. Most of the ponies fell silent ages ago. The few that are left are isolated and bound to these dreary halls. I do not think anyone really wishes for this to continue indefinitely."
"And what about the dragon?" Celestia inquired.
"A dragon?" Luna repeated, her eyes as wide as saucers.
"Ah yes, Spike. I am truly grateful to you for freeing him," Twilight replied. "I have long been aware of his presence, but we were quite powerless to help him." She gazed upwards. "Our poor little baby dragon, he's had such a hard life so far. But he's a big boy now. He'll be ok."
"He waited so long to talk to all of you again. Doesn't that mean anything?" Celestia insisted.
"He is saying his goodbyes now," Twilight revealed. "The others have already told him what's going to happen."
"This is just..." Celestia stomped her hoof. "No. There's no way I'm taking the Rainbow. It's just not right."
Twilight nodded. "That is truly selfless of you," she said as she stepped over to the pedestal. "This only further proves how fitting you are to be the next guardian of the Rainbow." Closing her eyes, she carefully set her horn against the glowing pendant.
"What are you doing?" Celestia demanded.
"Farewell," Twilight said. "Please remember to always use this power wisely and for the good of all ponykind."