For the remainder of the night, Luna made sure to be swift when tending to her regular duties, so that she could keep a keen eye on her sister. But the strange, evil demon named John Doe did not rear his head again for the remainder of the night, which only put Luna more on edge. The night's silence, previously a comfort, now only invited self doubt and stress, anxiety and discomfort.
Eventually, it was time for the moon to set. This meant that the sun would soon need to be lifted into the sky, sent on its regularly scheduled celestial journey. Luna dreaded the moment when when she'd have to alert her sister to her own failure... granted, her sister was a kind and benevolent one, but who in their right mind enjoyed disappointing a damsel in distress?
Celestia woke up, bleary-eyed and yet again exhausted. Hopefully Luna would have some good news. She painfully sat up in bed, only to see her younger sister trembling in the corner of her room, huddled into a fetal position and muttering "This is not good, this is not good, this is not good..."
Thus encouraged, Celestia tried to make her way over to comfort her, but found every step an arduous struggle. The pounding on her head increased, and she fell to the floor heavily. Her mental tenant was jack-hammering away at the walls of her mind, and suddenly everything was just pain.
She couldn't take it. She pleaded aloud to the malevolent entity inside her, to stop, to die, to fade, to escape, to do anything to lessen the pain. She just couldn't take it.
Her internal mental defenses came crumbling down, and the demon John Doe only gained a sinister grin, brightening up the dark oubliette he had created for himself to inhabit, within Celestia's mind. He could feel the constant pressure of her subconscious vanish as if it had never existed. He was temporarily taken aback by surprise; had her subconscious fortitude really fallen so quickly? He was only putting forth about fifty percent of his maximum effort in order to force the alicorn goddess to yield, but he wasn't too terribly worried. He was just pleasantly surprised. Amazing the kind of luck you can have when you do nothing but hate and discriminate.
Luna had been startled from her self-deprecating reverie only by the loud thump caused when Celestia's formidable body hit the floor hard. "Tiaaa!!" Luna galloped over to the princess of the sun, and held her now lifeless eyes up to meet her own. "What's wrong? What happened? John Doe, are you in there? Whatever you're doing, stop it! Stop it right now!"
A subtly dark aura began to envelop the normally bright white mare, and she began levitating off the ground by sheer negative energy. Her eyes opened to reveal only blackness and hatred. Luna only gave them a glance and nearly lost her sanity.
Her voice took on a deeper, echoing, sinister tone, as if the demonic force within her had possessed her completely. John Doe's warped voice mixed with Celestia's and uttered forth a terrifying proclamation. "Luna, you have failed to protect your sister. Her mental defenses have crumbled into nothing. I am in complete control, and you are the only one to blame, you pathetic little whelp. Never again will you hear your sister's voice, or receive from her a tender embrace. She is mine, and I say that her brain has lived on for long enough. So here's your last chance to say goodbye, Luna. I honestly don't even care about my life, or this land, right now. It's all nothing but nonsense anyways. I'm going to destroy this mind, and myself, and your sanity. And there's nothing you can do about it. Have a terrible life."
Celestia's eyes rolled backward into her skull, no pupils and irises left; just blank, white nothingness. She crumpled into the ground in a heap, shallowly breathing, but alive. Luna just broke mentally; she couldn't be strong anymore. Her sister had just been taken away from her for no bucking reason at all! She wrapped her hooves around Celestia's body, and mumbled sweet nothings into her ear. "No, you're all right, it's all right. It's going to be okay. You'll be fine tomorrow. This is just a bad dream. Nothing bad is happening. It's all fine. You're not gone. You're right here. You're my sister. You can't leave me. This can't be happening. It's just not fair. You'll be fine."
Her pointless meandering words trailed into incomprehensible nonsense. She was babbling forth, her thoughts all jumbled into inexplicable gobbledygook. A Royal Guard, hearing the commotion, crashed through the door to see what all the hullabaloo was about, only to be literally blown back by the force of Luna's Royal Canterlot Voice. "GET THE BUCK AWAY FROM US!" The guard was used to the voice normally, but this one was even stronger than usual. He flew at a breakneck pace backwards into a wall, crashing through and landing in an unconscious pile of limp limbs, out for the count.
Luna slapped Celestia. She poured buckets of water over her face. She spun her about with her dark blue magical levitation. She even tried tickling her, but nothing worked. Celestia would forever gaze, unblinking, uncomprehending, and absentminded, for the rest of eternity. Eventually resigned to defeat, Luna carefully set Celestia back in her bed, tucked her in, and closed her eyes.
Not knowing what else to do, she began brokenly singing a lullaby, pretending that her elder sister had only gone to sleep. Yes, she looked calm in bed. She was just sleeping.
"H-h-hush, n-now, quiet now, i-i-it's time to l-lay your, your sleepy h-head. H-h-hush, n-now, quiet now, it's time, it's time, it's time to go to... to bed.." Fat, salty tears ran down Luna's face like rivers of melancholy. Her runny nose sent mucus drifting down into her coat, and she tore at her mane with her hooves, large swathes of ethereal and flowing hair cast to the ground, now still and lifeless. Bald patches appeared on her head, and she ended up wailing like a banshee, lamenting the loss of her sister and, unknowingly, the loss of her peaceful mind. Forevermore she would be a turbulent loner.
Somehow, despite her current state of mind, she realized she would have to cover for her sister's disappearance from the public eye. Later that day she ruthlessly tortured one of the changeling prisoners which had been captured during their invasion of Canterlot earlier. It had taken them a while, but by now they understood the hissing, harsh language of the changelings. Luna eventually figured out how to adapt her own magic to transform her body into that of Celestia, and also used the magic to restore her appearance to one more befitting a princess. It might not be a perfect solution, but it would have to do. She couldn't trust anypony else with this task. It was her responsibility, and hers alone.
The next day the sun rose just as it always did, but it almost seemed more somber in a way, to all those who were awake at the crack of dawn. Somehow, even though it still shone with its regular warmth, everyone its rays touched felt just a little bit sad on the inside.
Luna stood on a castle balcony, focusing her efforts on dealing with two celestial objects at the same time. It had to be done. Even if her world had crumbled, it did not allow her to forsake her sister's subjects. A sad sigh escaped her lips, but this was the only evidence of emotion she allowed herself. She had to be strong.
A knock on the door drew her attention away from the sunrise and sunset. She made sure the day was well on its way before she went to answer it.
Twilight Sparkle stood at the door, her mane unkempt, her eyes twitching from lack of sleep, sunken into her skull. Clearly, something was worrying her.
"Hi, Princess Luna, I think there's something wrong with Princess Celestia! She won't answer any of my letters. What's going on?"
Luna took a deep breath, and prepared to begin a life of lies, all to carry on in her sister's memory. Her face became one to bear moderately bad news, a mask for the shattered and broken character within. "Calm down, Twilight, everything's fine. I'm afraid Celestia's just not feeling very well today. I'm sure it's nothing though; there's no need to worry. What she needs is just to be alone for a while. If she gets better, I promise I'll send you a letter as soon as possible. I'm certain everything will be fine. Here, let's go to the kitchens and get a cup of tea. I know it always makes me feel better."
Twilight took Luna's words not quite at face value; she detected that something was a bit off about the princess of the night, but discounted it as nonsense.
Over a warm cup of tea with milk and sugar, inside Luna's enormous quarters, Twilight attempted to make small talk with Luna, in order to decipher what was wrong. "So... how have things been lately? You seem just a little bit worried. Are you sure Celestia's going to be okay?"
Luna was staring off into the distance, only barely conscious of the conversation taking place. She blinked a few times and turned her attention back to her sister's former protege. "I'm sorry, I was just thinking about something. What was it you said?"
"I was asking how you're doing, Princess Luna. Is everything all right?"
"...yes, yes I suppose it is. I was just feeling... a little bit lonely. But it's quite all right. Nothing to be worried about. Please, don't tell anypony about Celestia's status. I'm sure it would just cause undue worry. Can I trust you not to worry, Twilight?"
"Sure thing, Princess! You can count on me."
"Thank you, Twilight." Luna gazed out the window, and it was as though she looked not at the land, but beyond it.
Sensing that perhaps Luna just didn't want to talk about whatever was going on, Twilight decided to give her some space. She figured that sometimes a bit of solitude was necessary to sort out one's thoughts. She politely excused herself from the table, made her way over to the exit, and parted with a final farewell. "Have a nice day, Luna! And tell Celestia she's got the support of me and all my friends, wishing her to get better!"
Luna quickly wiped a tear away from her eye before it betrayed her true feelings on the matter. Celestia wouldn't hear any well-wishing, or speak any kind words ever again. "Yes, I'll carry on your message. Have a... a bright day."
As soon as Twilight was out the door, Luna walked over to her sister and removed a few strands of hair, a bit of DNA to grasp onto in order to ensure the transformation process worked properly.
In a moment of bright light, Princess Luna seemingly vanished, and in her place stood once again the regal and royal Princess Celestia. Luna looked in the mirror, and try as she might, she couldn't pull off a genuine smile, the kind that warmed the heart like they should. She took a deep breath, though, steeled herself, solidified her will, and went outside the room to carry on in her sister's hoofsteps.
She had a feeling that this would be but the first of many long days. Many very long, sad, lonely, and melancholy days.