Luna stared at the small form in the hospital bed before her. As an alicorn, all other ponies appeared small to her, like children. Female chiropterans were slightly smaller than ordinary ponies, but not by much. The illusion seemed to be mostly because Cavern Melody was still asleep, but was augmented by Luna’s terrible realization about the fragility of mortal life.

Several days had passed since the incident. Cavern Melody’s condition had initially worsened, a result of residual magic retained within her body that her unknown savior could not absorb. Her breath had become shallow, and the re-formed elements of her skin started to break down. Luna had been terrified, but the doctors had been able to heal the new damage. There were still substantial, pale scars on much of her body, but Cavern Melody had stabilized.

All the time, Luna had neither slept nor eaten. She technically required neither. She felt hunger but could not starve, and instead of tiredness she only felt the shadows growing closer, even with glowing crystal on her neck driving them backward into the corners of her mind. She had instead sat by Cavern Melody’s bed, waiting. When Cavern Melody awoke, Luna wanted to be the first pony she saw, and to be the one to explain what happened.

Celestia had visited several times, mostly for her sister’s sake but also out of genuine concern for Cavern Melody. Celestia was not and had never been a fan of chiropteran ponies, but the sight of Cavern Melody’s scars still brought tears to her eyes. That had been before the messenger had brought the necklace, and the shadows had been at their strongest. Luna had seen into her sister’s mind, and seen her fear, not for Cavern Melody’s safety, but for Luna.

The necklace had eventually come, though. Luna had been uncertain, but was reassured of its safety- -both Pinkie Pie and Twilight Sparkle had tried it on, and Twilight had indicated to the messenger that there had been no effect on either of them. When Luna slipped it on in place of her own royal necklace, the shadows were instantly driven back and she felt much stronger.

Her sister had not returned since then, though. Celestia was busy with the situation at hand concerning the creature that had attacked the Crystal Empire. Luna was not a fool, though. She still remembered Cadence and Shining Armor’s descriptions of the creature, and she was aware of the similarities between it and what had visited her that night. A terrible, blue shapeshifter. They were almost certainly the same, or related creatures.

The creature itself was a paradox, though. It had attempted to steal- -or “destroy” the Crystal Heart, as Cadence maintained- -and had stated itself that its goal was to kill Luna. Yet instead, it had, at great personal cost, healed Cavern Melody. Not just healed, actually. Luna’s mind still nearly refused to accept it, but it had resurrected her. She had been dead, and it had pulled her back. Such a thing was even beyond Celestia’s power.

Still stranger, it had told her where to find a device to keep the shadows out of her mind. That made her wonder if the shadows were somehow damaging to it more so than her, if it somehow was afraid of them. She supposed that might be the case; after all, she had been able to mentally perceive it long before it reached her, a power that she now lacked with the necklace on.

As she considered these things, she suddenly saw Cavern Melody stir.

“Cavern Melody,” said Luna.

“Your highness,” whispered Cavern Melody, her voice as beautiful as ever but terribly weak. She opened her eyes, and Luna saw that one of her beautiful red eyes had been replaced with one that was ghostly blue. Tears suddenly welled in both eyes. “I saw…I saw the Deepest Darkness,” she said. “I- -I was there. I heard their singing- -but something pulled me back. Why? Why would you pull me back?”

“It was not I,” said Luna. “But you cannot leave me yet. Not when Nightwatcher needs you. Not with the foal.”

“The foal!” she cried, suddenly, trying to sit her up.

Luna gently pushed her back down. “The foal is safe,” said Luna, her eyes wandering to Cavern Melody’s midsection, which even below the white sheets was visibly starting to swell. Inexplicably, the foal within had grown substantially, far faster than a normal fetus was supposed to. “The doctors were actually quite surprised by her health and vigor. I am not, though, as it is a product of both you and Nightwatcher.”

“Her?” said Cavern Melody. “I’m having a filly?”

“Yes,” said Luna.

Cavern Melody smiled, and leaned back into her pillow. “A filly,” she said. “I will need to inform Nightwatcher.” She sat up again.

“No,” said Luna. “You need to rest.”

“I suppose you are correct, your highness,” said Cavern Melody, leaning back reluctantly. “Although, admittedly, I feel quite vigorous.” She looked down at her hooves and, for the first time, noticed the scars that twisted over her body. Strangely, she did not seem to be disturbed by them. Her disfigurement actually seemed mildly amusing to her.

“Cavern Melody,” said Luna. Now she was starting to cry- -both tears of joy, for seeing that her friend was healthy, and tears of deepest sadness, for knowing that she herself had been the cause of her misfortune. “I am sorry. I am so, so sorry. I nearly killed you…I thought I lost you…you have every right to be angry.”

“I am not angry, majesty. I am actually quite honored that you have visited me during my recovery.”

“How can you not hate me?” said Luna, now sobbing. “After what I did to you…”

“You were well within your rights. By tradition, as your servants, we are prepared at all times to be sent to the Deepest Darkness by you, for any infraction or none at all “

“How can you say that?”

“Because that is the truth. Now look at me.” She raised her foreleg, showing the barely healed burns that covered it. “I bear the mark of your divine glory, my princess. None living can claim this honor.”

“But the foal…”

Cavern Melody sighed, and turned away from Luna. “I will admit,” she said. “When I became aware that I would not survive, I was saddened. Please do not be insulted, but I was sad that my child would never see life, and that I would leave my one love, Nightwatcher, alone in this world. But I am not like you, majesty. I am only mortal, and cannot help but think such things.”

“You have every right to think them,” said Luna, taking Cavern Melody’s hoof in her own. “Please, Cavern Melody. Your life is more valuable than your function as a servant. Do not disparage it. I- -I was so afraid that I lost you.”

“Princess…”

“You are not just my servant. You are my friend. I can never forgive myself for what I did to you.”

Cavern Melody sat up suddenly, and wrapped Luna in a hug. Luna tried to say something, but the words stuck in her throat. All she could do was hug Cavern Melody and cry.

They eventually released each other, and Luna smiled. She felt somewhat better, and the shadows in her mind had almost completely vanished.

“I am confused, though,” said Cavern Melody. “If you did not call me back, who did?”

“I do not know.”

“What kind of a being would be cursed with such a terrible power?”

“I am unsure. But…”

“Something is troubling you, princess.”

“Absorbing my magic from you was…catastrophic for his body. I am sure he survived, but before he left, he said something.”

“And what was that?”

“He spoke of a ‘night-dance blade’.”

Cavern Melody’s happy expression suddenly faded. Her eyes narrowed, and all emotion vanished from her face. “Ah,” she said. “So you know.”

“Know what, exactly?”

“What my cutie mark actually is.”

“But your special talent is for singing.”

“No, it is not. You misunderstand the meaning of my name.”

“Then what does it mean, if not a song?”

“It is a song, but a ‘cavern melody’ is not a song that one can sing more than once. It is the echoing, musical final screams of one who has met a violent fate in the depths of the caves. Less poetically, it means ‘death song’.”

Cavern Melody turned toward Luna. Here pupils focused, and Luna saw that her blue eye did not have a vertical slit for a pupil, but rather a simple black hole in the center, like an ordinary pony eye. “Princess Luna,” she said, flatly, but with just a hint of shame. “My special talent is for murder. I am an assassin.”

“But…but why?”

“Why was I born as such, or why I kept it from you?” she sighed. “I suppose I knew that someday I would need to answer these questions. But perhaps I was content to be a simple servant, and your friend. None can change their fate, though.”

“This isn’t possible…”

“No, it is the truth. But in my pride I kept it from you. Because I know you. What use do you, a loving and truly caring princess, have for a murderer? Would you send me away if you knew what I was?”

“I would never!”

Cavern Melody smiled, and for the first time Luna clearly noticed her fangs. “Thank you, princess. You do not realize what that means to me. I am truly glad that I was of the generation alive upon your return.”

“My return,” said Luna. “You mean you served me before?”

“Myself, no, but my ancestors have served you since the time of Nightmare Moon, and even before that, in secret. There is no record of us, aside from the greatest monument to one of my ancestors- -a tiny scar between your sister’s wings.”

“You…you tried to kill my sister.”

“Not me, personally, no. My ancestor, who was killed for his greatest glory. He struck at your orders.”

“Not my orders,” said Luna, perhaps too loudly and too angrily. “At Nightmare Moon’s!”

Cavern Melody smiled and shook her head. “You misunderstand me,” she said. “Myself, and my people, we will follow the orders of either Queen Nightmare Moon or Princess Luna. The words of either are equivalent to us, and we love both equally.”

“How can you say that to me?” said Luna, her peace of mind shattering. The shadows began to return, and now she could feel them staring at her, waiting. “Nightmare Moon was a monster, a creation spawned by my hatred and jealousy for my sister. How could you say such things about her?”

Cavern Melody only smiled, and once again took Luna’s hoof. “No, majesty. You do not understand. To us, you two are opposites, but not opposing. Two halves of the same being. Nightmare Moon was arrogant and violent, but powerful, protective, and righteous in her own way. When our people were persecuted and hunted, she was the one who protected us, and promised us, her chosen people, eternal night. You are her opposite. You hold the love, kindness, and understanding that she could never know.”

“I don’t understand,” said Luna. She had accepted that Nightmare Moon was a part of her, but her darker half, an incarnation of her internal darkness brought to life. Her memories from that time were fragmented at best, but she remembered brutality and destruction at her own hooves.

“Two halves to make the night,” whispered Cavern Melody. “The black of the sky, the light of the moon. To fear the darkness, and revel in the beauty.” She appeared as though she was going to say more, but here eyelids suddenly seemed heavy. She leaned back into her pillow and, with a soft moan and a smile on her face, went to sleep.

Luna watched her for a moment, and then gently pulled the covers of her hospital bed to better cover her exposed neck. Cavern Melody turned and scrunched the covers in front of her, hugging them close to her. Luna stood and, for the first time since Cavern Melody was struck down, left her side. She was hungry, and intended to go to the kitchen and ensure that there was plenty of food for Cavern Melody when she awoke while she got herself a snack.

Her mind was troubled, but the shadows had received somewhat. The glow of the necklace still kept them away, its strange energy forcing them away even as they gathered in the gloom of Luna’s shame concerning Nightmare Moon.

A light illuminated the darkness of the hall, a familiar warmth. Luna looked up from her intense thought and saw her older sister approaching, smiling through her concern.

“Luna,” said Celestia. “You are…out. Has Cavern Melody’s condition improved?”

“Yes, dear sister,” said Luna, smiling. “She awoke and we spoke, but she is resting again. I was going to the kitchen to bring her some food for when she wakes up.”

“That’s very sweet of you, Luna. I was just there myself. The cooks have just started with the fall flavors of ice cream, and there may be some left if you…” her speech trailed off, and her eyes focused on Luna’s necklace. “Where did you get that?”

“It was as gift,” said Luna with equal parts defensiveness and concern. “I rather like it.”

“That is not something you should have,” said Celestia, her smile vanishing completely. “That is not something anypony should have.”

“I am aware of no law banning such items,” said Luna. “And as a princess, I am- -we both are- -above the law.”

“Give it to me,” said Celestia, stepping forward.

Luna recoiled in panic. She knew what would happen if it was taken from her. Although ominous and somewhat ugly in construction, it was the only thing keeping her sane.

“No, sister, I need it!” she said.

“Give it to me!” roared Celestia.

Luna felt the warmth of her sister’s magic surrounding her neck. “No!” she screamed “It’s the only think keeping them from getting- -!”

The clasp of the necklace opened, and it was detached. Luna felt its protection collapse, and the shadows suddenly became visible. The voices rushed into her mind, consuming her thoughts, and she saw a pair of turquoise eyes flowing toward her on black smoke.

Celestia watched as her sister’s eyes widened. Luna jumped back, shaking, looking terrified.

“No!” she cried, igniting her magic and firing randomly. “No! Don’t let them take me!”

“Sister, what’s wrong?” said Celestia, herself suddenly terrified. She could not see the enemies that Luna believed herself to be facing.

“Sister?” she called. “Sister, please, help me! Celestia!”

Luna ducked back into a corner and released a low scream of pain. She crouched and seemed to convulse and, in horror, Celestia watched as the black stain that surrounded Luna’s cutie mark began to expand. It stretched and clawed its way over Luna’s body, pulling itself along by tendrils of shadow, dying her blue, hairless skin black

The magic around Luna’s horn shifted and changed, its color changing from deep blue to pure black. Then, suddenly, Luna stopped shaking. Half her body was now black, with only her front half free from the infection.

“Luna,” said Celestia, approaching her slowly. “Luna? What’s wrong?”

Luna’s eyes flashed open, and Celestia knew that she was no longer speaking to Luna. They were pale blue, with vertical slit pupils. The eyes of Nightmare Moon.

“And you dare to call her sister,” said the half-Nightmare Moon Luna hybrid, sounding oddly calm. Her voice was not filled with the arrogance that Celestia remembered, but was just as cold as ever. “After you take from her the only object that was keeping her sane. Even now she screams within me, in fear and confusion, and rejects my strength.”

“Nightmare Moon,” said Celesita. “I defeated you once, I can defeat you again!”

“Defeated me?” said Nightmare Moon, smiling humorlessly. Aside from the eyes, Luna’s face was still her own, but the expression was one that could only come from Nightmare Moon. “You watched as your sister filled me with her pain and jealously. I took what she could not bear, and grew strong on hatred and misery. And rather than cure her with love and kindness, you reacted as you always do, with violence. You banished her to the moon, where it was me who kept her mind intact for one thousand years.”

“What do you want?” said Celestia, charging her own horn but knowing fully well that she could not strike down Luna, even if she was partially converted to Nightmare Moon.

“Want?” said Nightmare Moon, as if the question was childish and useless. “I seek nothing except the protection of my host. Something you now deny her, for the third time.”

“Third?” said Celestia, her magic freezing.

“Yes,” said Nightmare Moon, forcing Luna’s mouth to smile. “I still have the memories that she does not. The first time she screamed, and called out for you, and you never came. You were blinded by your search for power. They took her wings, her eyes, her soul, and her life, and all the while she held out hope that you would come for her.”

“I did come for her!” cried Celestia, tears running down her eyes. “I was the one who saved her!”

“Really? Or was it that you needed a soul connected to your own to enable you link to the sun?”

“NO!” screamed Ceslestia, the colors of her mane fading into pure white. Her light forced Nightmare Moon back, and with her magic she gripped the necklace and thrust it toward Luna.

A thread of dark-colored magic caught it before it reached Luna, holding it back.

“Before I depart,” said Nightmare Moon. “A gift for you, as I gave you twice before.” Her eyes widened and seemed to dimly illuminate. “The destroyer who has watched through the ages revels in his trickery. The betrayer Choggoth shall rise, and what was once his master becomes his servant, his body, and he shall seek his heart. The true defender of Equestria will stand alone against him, and shall fail!”

The necklace snapped forward, its ends clasping around Luna’s neck. The shadow material consuming her body retracted to its residence on her rump, and she blinked. Celestia saw that her eyes had returned to normal.

Luna collapsed to the ground, and Celestia caught her as she fell. The exertion has rendered Luna semiconscious, but she was weeping. “Sister,” she moaned. “Sister, help me…”

While Luna was resting, Celestia found herself in the depths of the castle, walking through ancient stone halls that none but her had ever passed through, the only guiding like the tiny sphere of light above her horn.

The situation was going poorly. Luna’s mental state was decaying rapidly, and Celestia had no explanation for why. She assumed that it was likely due to the influence of the Choggoth, which she had actually seen attempting to attack Luna. What Nightmare Moon had to do with the equation was unclear, though.

What was clear, though, was that the Choggoth was more of a threat than Celestia had dared to imagine. It had somehow bypassed all of the castle’s security and entered her sister’s bedchamber. Celestia did not know what a Choggoth was actually capable of, but if it was in possession of cerorite, it surely could have killed her while she slept.

Her actions so far had failed against it. Her Light, it seemed, had failed her. They had not yet returned to the castle, but Celestia had received a letter from Twilight Sparkle indicating that two ahuizotl had attempted to foalnap three fillies. Not only that, but in the process they had attacked Rainbow Dash. As if Celestia did not have enough burden on her conscious worrying about the trauma and of three of her youngest subjects, her Light had attacked the Element of Loyalty. If they had used a fully poisoned dart, Rainbow Dash would have been killed, and the Elements of Harmony would no longer be functional. Not only that, but Rainbow Dash was still young, a Pegasus with prodigious flying ability. She had a bright future ahead of her. Celestia could not have forgiven herself, ever, if such a life were to be ended on her orders, even collaterally.

Worse, the Choggoth- -or “D27”, as it had come to be known- -was still suspected to be alive. Celestia knew that it would likely return to Ponyville, where it felt safe and had probably manipulated various ponies into protecting it. Sending in an army was still out of the question, though, at least immediately. If she sent spies or soldiers and it had not yet returned, it would surely detect them. It would hide, and she would never find it. It would fester and grow like a cancer.

She had a plan, though. Sending troops and guards to Ponyville would alert the Choggoth only because they were abnormal, something it did not recognize. Fortunatly, Celestia already had one of her top lieutenants present in Ponyville. She had sent a carefully crafted letter to Twilight Sparkle, ostentatiously asking her about Luna’s necklace, which she already knew to be a highly modified power core from a disruption rifle. She had asked if Twilight had seen the necklace’s creator recently, stating that it was a blue, unusual looking pony.

When Twilight responded, it would signal that the Choggoth had returned, and it was time to begin the attack.

Which was why Celestia had come to the forbidden halls of her own construction. This was not an enemy that she could fight alone, and not one that could be defeated by normal means. It was a fragment of a forgotten age, a time when things were far more powerful and dangerous. As such, the only way to defeat it was to use techniques and weapons from the past to fight it.

Celestia reached the final chamber of the halls. It was a single, massive cavern that she had carved from stone almost four thousand years earlier. In the center sat a massive cubic structure that nearly filled the room. It was a vault, built from metal that could not be pierced by any known kind of magic or weapon, at least not easily. Celestia had built it herself long ago to contain the items that she hoped she would never need to use again.

At the front was a door with no knob or handle. Instead, it had a small hole in the center. Celestia sighed, and braced herself for what she knew would happen. She took a breath and inserted her horn into the hole.

The shock was incrediably as her magic fed into the system and was immediately forced back into her. Her mind itself flashed with confusion and images, and she cried out in pain as she collapsed to her knees, her horn slipping out of the hole.

She gasped in pain, the her own magic arcing around her body. The feedback had been purposely amplified by the machine and converted into a form that was designed to kill any unicorn who attempted to unlock it. Only Celestia’s magic could truly open the box, and only she was able to survive the blow it produced.

The door slowly shifted, and the metal of the complex lock retracted, the gears within clanking and whining. Celestia slowly rose, shaking, and watched as the door parted in three directions, releasing the stale air from within.

She had sworn to herself long ago to never open this box, and, for the second time, she had broken that promise. She entered slowly, and stared at the contents. Lining the shelves were the bad memories of what she had once been, and what the meaning of life in Equestria had once been. Filling every shelf were weapons of every type. There were energy rifles, disruption cannons, firearms, mortars, particle beams, and thousands of other terrible devices, all still perfectly viable and undecayed even after five thousand years.

Celestia passed the seemingly endless shelves. She wondered to herself how many ponies had been felled by these very weapons, but decided to ignore that thought- -far more ponies, she knew, had fallen to the magic of her own horn.

This was the legacy she had spent her eternal life attempting to erase. A legacy of endless war and combat between the races of ponies that had only accelerated as time went on. Celestia herself had been a part of that war- -and she recalled just how those rifles felt in the grip of magic, or what it felt like to be shot with one.

She rubbed her front left leg, as if it still hurt, but only stopped for a moment. She instead made her way to the farthest back shelves. There, displayed amongst the worst of the abominations in the vault, was a suit of pure white armor. It was not armor like the mostly useless ornamental forms that her guards wore, or even like the older iron medieval forms that she had always found so amusing. It was thick, heavy, and made of substances that nopony even knew how to manufacture any longer. It was armor that could never be worn with pride, but only with shame; it had not been built for beauty, but to surround a bringer of divine punishment.

Celestia took off her shoes, her necklace, and her crown and set them on the ground. She stood naked before the armor. Then, slowly, she raised the first piece of it in her magic and began attaching it to her body.