The train soon arrived in Canterlot and Spike and the ponies approached the castle gates. Two royal guards were stationed in front, barring their way.
"The Princess is unable to see anypony," said one of the guards. "I'm afraid not even you, Princess Twilight, may enter."
"Ho, guards!" Princess Luna stepped out of the castle. "Let them in!"
"But... Princess Luna, we must refrain..."
"They are my sister's most loyal subjects," insisted Luna. "They have to know."
"As you wish, your highness," said the guard, as he and the other guard stepped aside. "But it might not be wise."
"That's for my sister to decide." Luna led Spike and the Mane 6 into the castle. "I'm sure you saw what happened earlier."
"Everypony in Ponyville saw it," said Twilight. "They must have been woken up by the sudden brightness. Do you know what happened?"
"Yes. It started early this morning when my sister tried to raise the sun. Something terrible happened..."
In a flashback, Celestia stood on her balcony and started using her magic to raise the sun, lifting herself into the air as it rose. But all of a sudden, Celestia was struck by a sharp inner pain right in the heart, widening her eyes and gritting her teeth. The sun violently rose into its highest position in the sky as if responding to Celestia's irregular heartbeat. Celestia fell back down on her balcony and tried to stand up. She managed to stagger back into her chambers, but collapsed quickly. And at the same time, the sun quickly went down in the same direction it rose.
"S... sister..." Celestia uttered through limited breathing. Princess Luna arrived on the spot.
"Sister!" she cried. "Someone help!! MEDIC!!"
"Y'mean the Princess had a heart attack?" asked Applejack.
"I fear it's much worse," said Luna, deeply saddened as she led the group of friends through the castle corridors. "Her heart condition was temporary, but it paved way for her to contract... a terminal illness."
"T-t-t-terminal...?" Twilight stammered in horror. "You mean... she's dying?"
"I'm afraid so," replied Luna, tearing up. "The worst part is, we don't know what kind of illness this is. Even our unicorns with their most advanced healing magic are only barely able to keep her alive... but it's all they can do."
"Oh, this is terrible," said Fluttershy. "Is there anything we can do?"
"I'm not sure. But I'm certain my sister will be glad to see you. I would have told you to come myself, but I couldn't bear to leave my sister's side. But I knew you would come, so I waited for you."
"Wait..." said Rainbow Dash. "How do we know this illness isn't Discord's doing?"
"Because while Discord may be a prankster, I'm almost certain he would never intentionally claim any lives. Otherwise, he wouldn't take so much delight in any havoc he creates if there's no one to react to it."
"Look, Rainbow," said Fluttershy. "For all we know, this whole thing could have just happened. I don't think anypony did this to the Princess. Maybe she was just stressed out and worked herself too hard."
"Are you kidding?" replied Rainbow. "She's ruled Equestria for thousands of years! How could things possibly be stressful for her?! And even if they were, why now? I still think Discord is doing this just to 'test our friendship' for the umpteenth time!"
"Rainbow!" snarled Fluttershy. "That's absolutely ridiculous! And even if Discord is testing our friendship, it's because he's making a point!"
"What do you mean?"
"I've seen the way you girls behave around him. Whenever he shows up, you've never been happy to see him! Whenever he's around, you all accuse him of causing trouble! You know, maybe he's right! Maybe you girls are using him just to make sure he doesn't cause any mischief simply because we don't have the Elements of Harmony anymore! We have no proof that Discord is behind this and no hard evidence against him!"
"That's it!" Rainbow shouted. "I'm tired of this! If you'd rather take Discord's side and give us all sideways glances, then maybe you'd rather be friends with him instead of us!"
"Rainbow, shut up!" snapped Rarity, surprising the others. After all, it wasn't every day that such a refined mare like Rarity would resort to saying something so powerful. "I'm sorry I had to say that, but you're just being stubborn and unreasonable! Fluttershy is our friend too! Her standing up for Discord should not have to change that." But Rainbow just rolled her eyes, earning her another angry stare from Rarity.
"I get it, Rarity," she said. "But even if Discord isn't behind any of this, how do you explain the sky being stuck in... well... like that?" She pointed out a window where the sky was still shown to be frozen in twilight colors. "I doubt Celestia's illness is connected to that."
"It's not." said Luna. "I tried to raise the moon to test that theory. I couldn't pull it above the horizon."
"And Discord tried to bring back the sun," said Fluttershy. "But it wouldn't stay in the sky."
"And whoever caused this besides Discord just 'happened' to do it at the exact same time the Princess starts dying! Sure. I'll buy that." Rainbow's voice dripped with sarcasm.
"Let's just go visit my sister for now," said Luna, surprisingly having put up with the Mane 6 arguing. "The important thing is, we are at least able to keep her alive... for now. But she is suffering from a great deal of pain."
The group arrived at the medical wing where Celestia was being treated. Two guards blocked the room, not about to let anyone in, unless it was Princess Luna.
"The Princess is in absolutely no condition to see visitors," said one of the guards.
"That will be for her to decide," said Luna. "Now stand aside!"
"As you wish, your highness," said the other guard, both of them stepping aside to allow the group to enter the room. Princess Celestia was in bed, hooked up to an oxygen mask with computers monitoring her current heartbeat, which was surprisingly in stable condition despite her illness and labored breathing. She looked even paler than white and her eyes showed that she was not sleeping, but going through intense periods of pain. Her royal garments that she usually wore were all cast aside. Four unicorns with highly advanced healing magic were all casting spells on her at the same time, preventing the terminal illness from claiming Celestia's life. A stallion doctor approached Luna.
"It's not good, your highness," said the doctor. "She needs treatment, and fast."
"I know," said Luna, beginning to cry.
"This is terrible," murmured Twilight. "What do we do?" Luna went to her sister's side, so Twilight decided to do the same.
"Sister..." Luna whispered. "Princess Twilight is here to see you." Celestia barely opened her eyes.
"T... Twilight...?" she moaned. "Is that you...?" She tried to raise one of her hooves to comfort Twilight.
"No, sister!" protested Luna. "Save your strength! You need to rest!"
"I'm... fine... Luna..."
"You most certainly are not! You are dying!" It pained Luna greatly just to say that. Twilight held Celestia's hoof with both of her front hooves, on the brink of tears.
"Oh, Princess..." murmured Twilight, afraid if she said too much she would burst into tears.
"It's alright... Twilight..." Celestia whispered. "I'm glad... you're here..." Rarity spoke to the doctor.
"Doctor, please," she said. "Is there anything we can do?"
"I know she needs treatment," replied the doctor. "But I don't know what can cure this kind of illness."
"Maybe Twilight could look it up in the library," suggested Spike, wanting to be hopeful.
"That might be our only hope, Spike," said Applejack. "Twi's usually good about that." She looked over at Twilight by Celestia's side.
"Your friends..." Celestia croaked. "They came... Let them come... to me..." Twilight continued to hold Celestia's right hoof as the others came to her cautiously. "I want you all... to know..." said the dying Princess. "If anything happens..."
"Don't say that!" said Twilight, tears streaming down her face.
"Listen to me!" said Celestia. "If anything happens... to me..." She panted through her oxygen mask. "I want you... to know... I am proud... of everything... you have done... for Equestria... for me..." Applejack removed her hat and shed her own tears. "And Twilight..." Celestia huffed. "I want you... to know... I'm sorry... for doubting you..." Twilight knew exactly what Celestia was talking about.
"No, Princess!" she protested. "Please don't bring that up! It was--"
"My fault!" heaved Celestia. "It was... my fault... I'm the one who let you down... Me... and your friends... You did... nothing wrong... And... never... ever... say that... I was flawless... Promise me that..."
"But, I..."
"Promise me, Twilight... Promise me... you will acknowledge that I have made... my foolish mistakes..." Twilight had no choice.
"I promise, Princess," she wept.
"Oh, sister," Luna cried, nuzzling Celestia's head gently.
"I love you, Luna," said Celestia, finally shedding tears herself. "I love you all... Thank you... for everything..."