"This isn't right." Luna said bluntly, her voice echoing across the empty throne room. "I can't do this."
"Luna!" Nightmare Moon asserted. "Get a hold of yourself! Nopony said this was going to easy!"
"It would be murder, though!"
"It's for the good of Equestria, and for a million innocent pony lives! You won't be able face him when he comes to power, so do it now!"
Luna looked straight up, locking her sleep-deprived and bloodshot eyes with those of the God of Chaos. She saw no activity in his lifeless glare, and that somehow made what she was about to do all the more terrifying. He had no defense to any attack, all he could do was watch as his life came to an end.
"Close your eyes, and get it over with." Nightmare Moon cooed softly. "Leadership isn't always about being a hero, Luna. Remember that."
Luna nodded, and continued looking up at Discord with a pleading expression. She stared at his stone body for what must have been a minute, but it very well could have been an hour and she probably wouldn't have noticed a difference.
"D...Discord...I know you can hear me." She eventually said, ripping off her hood and cloak. "I don't want to do this...but—"
Suddenly, a loud, cracking sound exploded from the stone, and a sizable tear began spreading across his body, bleeding out blinding white light.
"The Elements are failing!" Nightmare screamed warningly. "Hurry, Luna! Do it now!"
"But..."
"DO IT!"
"Discord, I'm sorry!" Luna shrieked, squeezing her eyes shut as she charged her magic. She moved herself into a standard offensive stance, which was a half crouch, with her rump angled upwards and her head closer to the ground. "I'm sorry I'm not the mare my sister was."
Luna let out a pained wail as she unleashed her magic, hearing the shatter of the stone in front of her. She didn't hear Discord scream, there were no more blinding lights or deafening noises, all she heard after her own voice was the sound of the rock exploding and pattering against the marble floor. Still, even after all the sound was gone, she kept her watering eyes closed, as she fell to the ground panting. She had not even realized that she had been holding her breath.
"There! All done!" Nightmare Moon said, sounding almost completely uncaring about what had just happened.
"I'm a monster." Luna groaned, burying her face in her hooves and still keeping her eyes shut tightly.
"Well, yes. But this isn't the reason why."
Slowly rising to her feet once again, Luna used her hooves to rub her eyes dry, before beholding the scene in front of her. Discord, or, what used to be Discord, had been completely obliterated, reduced to powdery dust or the occasional chunk of rock.
Exhaling loudly, Luna blindly grabbed her cloak and hood, trotting up to her throne and placing both across one of its wooden arms. There would be no need for her disguise where she was going now. With Discord dead, all her business in Equestria was now complete. Even if she were to fail, her subjects would at least be safe from him, and from herself.
No matter what the cost, she had to at least try to bring the sun back, and this was simply the only way she could do it. She could never change what she had done, but perhaps she could at least redeem her past sins. In the end, a mare could only hope.
"I know what you're planning." Nightmare Moon sternly muttered, as Luna turned to face the sprawling expanse of the throne room, her horn once again sparking to life with dark magic. "I always have. I can't stop you, but I still want you to know that this is an extremely stupid idea. You're going to get us both killed."
Luna said nothing in response as she focused on casting her spell. Despite it being a largely illicit form of magic, and for obvious reasons, Luna loved the feeling of dark magic as it coursed through her, flowing as smoothly as water down a swiftly rushing river. Everything considered, all she was really doing was casting a particularly advanced teleportation spell, not dissimilar to the one she had used to teleport herself to the moon, it was more a matter of her destination that made this one of the most notoriously evil spells conceived by ponykind.
And her she was using it to save the world. In all honesty, Luna found it hilariously ironic.
"You may think you have the power to do this but I'm telling you right now, this will turn out to be your last, greatest mistake. And considering you killed your own sister, that's saying a lot." Nightmare Moon continued, her tone becoming harsher by the second, until she was all too reminiscent of the sickening voice that had first lured Luna into corruption. "Stop while you can and deal with this realistically, instead of stooping to such outlandish extremes."
"What's the matter, Nightmare Moon?" Luna cackled, the first traces of magic sputtering from her horn. All around her, various articles were now being blown about as if being caught in a furious tornado in the middle of the throne room. "Are you afraid my plan might work after all, and you'll once again be nothing?"
"Never. Do what you please, you fool." Nightmare responded, although the fear in her voice was unmistakable. "For a second I could have sworn you'd started showing signs of responsibility and leadership. I guess that makes me the fool, for ever giving you that honor. I'll enjoy hearing you crying your last regrets in the depths of Tartarus, "
Papers, dust, and even Celestia's funeral bouquets were all whipping around, still caught in the vicious tornado as Luna focused harder and harder on casting her spell. The spell's target in the middle of the throne room was beginning to take form now, slowly building from a pinprick of dark purple into a swirling vortex of the same colored magic. The light of the magic was now bright enough to illuminate the whole room, so that everything took on a purplish hue.
The portal was now more than fifteen feet in diameter and growing. Luna watched with fascination as even heavier objects such as her throne gravitated towards the now massive cavity in the middle of her floor. First, lighter objects such as paper and Celestia's flowers were pulled through the portal, sometimes vanishing and other times bursting into blue fire. Next, Luna's cloak and hood took flight and promptly exploded upon entering, their ashes visible for a split second before appearing on whatever other side lay beyond. Cracks began appearing on the marble floors, and Luna's racing mind quickly deduced that if she did not finish casting this spell the whole castle might be destroyed, or even worse; pulled through the portal. At the same time though, she couldn't preemptively leap through when the possibility of her appearing on the other side as ash was so prominent.
Trying to push the image of the paper and fabric burning out of her mind, Luna took a shaking step towards to portal, noting that she was the only thing not caught up in its gravitational pull. Even tree branches were crashing through the painted glass walls and disappearing, along with sizable bits of mortar from the already destroyed castle walls. Deciding she could wait no longer, Luna closed her eyes and ran straight towards the portal, screaming an excited battle cry and unfurling her jet-black wings as she took to the air for a brief second before vanishing from Equestria all-together, the portal instantly imploding into nothingness as soon as she did.
"That's it!" Nightmare Moon moaned as she and Luna immediately teleported away from the throne room and into complete darkness. "We're dead! Nice going, hero!"
"We're not dead!" Luna snapped, feeding magic into her horn in an attempt to light her surroundings. Even after pulling off such a complex dark magic spell, light magic was a cinch for the Alicorn of the Night, and in less than a second Luna's horn was glowing brightly.
And illuminating absolutely nothing.
"What?!" She exclaimed, boosting her magic and still seeing absolutely nothing. She could have been in the darkest room in Equestria during a lunar eclipse and she still should have been able to see SOMETHING! It was completely impossible for there to be literally nothing around her...
"Right about now, guards are probably storming into the throne room, undoubtedly investigating that weird scream you let out... Nightmare Moon mused. "I imagine there's a nice, big pile of ashes in the middle of the floor, along with a charred tiara..."
She continued complaining, although Luna wasn't listening. Instead, she raised a hoof up to her eyes, hoping to all that was holy that she would be able to see it. Thankfully, she could; her boots must have been lost, because all she could see was the blue of her hoof—
"Wait...blue?!"
Luna blinked in shock, half-expecting to open her eyes and see her hoof the same jet-black it had been ever since she had transformed. But, no, it remained the same dark blue that it technically was supposed to have been. There was not enough light to see the rest of her body, but Luna was willing to bet that she had once again gained her former appearance, before she had changed into that terrifying, dark parody of herself.
Fueled by optimism, Luna put her hoof down in front of herself and, shaking with anticipation, took her first step into the abyss of darkness.
After only fifteen minutes of walking, Luna already felt panic slowly start overtaking her short-lived hope. This darkness was indeed unnatural, most likely some sort of trap or prison, so what made her think she could simply waltz out of it?
But that wasn't necessarily what was making her uncomfortable. The darkness, she could easily have dealt with, but the silence was something else. Luna couldn't think of a time when she had been completely unable of hearing ANYTHING, no matter where she was, there was always the sound of wind, her footsteps, birds...something! But here, in addition to complete darkness, there was absolutely no sound...it was, in fact, so silent that after several minutes, her heartbeat started growing in intensity until it pounded in her head like a drum.
Even the ever-vocal Nightmare Moon had gone completely silent, and for once Luna felt like begging her to say something. In the end, though, she decided she would much rather go mad than have to stoop to the level of asking for that fiend's presence.
Finally, Luna could take the silence no longer. Against every notion of what she considered safe, given her location, she called out.
"Hello? Is there anypony out there?"
Nothing. No response, not even an echo. Dejected, Luna slumped to the ground and buried her face in her hooves.
"...Life..."
Instantly springing to her hooves, Luna whipped around frantically, trying to find the source of what she could have sworn was a pony's raspy whisper. The voice had sounded...desperate...or perhaps in pain. It was a whisper not out of choice, but out of complete and utter weakness. It had also sounded incredibly far away...if it hadn't been for the otherwise complete silence, she couldn't possibly have been able to hear it. Luna perked her ears and sunk slowly back onto the ground, listening desperately for the sound to come again.
Minutes passed, and still, nothing. With a mighty sigh, Luna once again rose to her hooves and began walking. She reasoned that there probably had not even been a voice, it was just her mind playing tricks on her. It wouldn't be the first time she heard voices in her head, after all.
"I smell...life..." The whisper came again, although this time it was much louder...or, more accurately, much closer.
"Who speaks?" Luna said, turning around to face the direction it had come from; behind her. "My name is Luna, and I mean you no harm! Please, do you know a way out of here?"
"It has...it has so much life. It glows. Can you not see it?"
"W...what...?" Luna gasped, once again illuminating her horn and shining it in the direction of the voice. For a split second, a black shadow of what looked like a unicorn whisked by, but it was just as soon replaced by the solid darkness. "I...I glow? Nightmare Moon, do you know what's going?"
There was no response from her companion. Luna began backing away fearfully, too frightened to stay still but also too frightened to turn away and leave whatever creature was stalking her out of sight. It was close enough now that Luna could hear its breathing, but even that sounded off. It was as if it was always inhaling, as a hound on the hunt would, but it never once completed its breathing process with an exhale.
And still the darkness reigned! Whatever creature was following her was obviously having no difficulty, but here she was tripping over her own hooves in the dark like a fool. Every time she fell down, she instantly sprung up again and continued backing away, flaring her light and praying that it would capture something.
"Feed!" The creature sounded like it was yelling now, its raspy words escaping from a throat that sounded as if it had not been used for decades. "Please! Let me feed! It must let me feed!"
"Feed on what?" Luna screamed in response, surprised even by the unfamiliar shrill of her own terrified voice. "I don't have food, I don't know what you're talking about! Nightmare Moon, are you there? For the love of Equestria, speak to me!"
Once again tumbling to the ground, Luna yelped in surprise as something once again crossed through the light of her horn, standing right on top of her. She again flared her magic and, this time, was greeted to a full view of the pony that had been chasing her.
And it was enough to make the Princess of the Night start in surprise.
"What in TARTARUS is that?!" She screamed, casting shocked glances backwards as she suddenly felt the urge to sprint away through darkness. It wasn't that she was frightened, but the twisted wreck of what surely could not be a pony was all the motivation she needed to properly find a way out of wherever she was. Behind her, the thing had stopped forming words and was now groaning and wailing after her, but thankfully her increasing distance away from it was slowly masking its pleas. There were no changes to the void around her, and yet still she continued running, wishing desperately to be anywhere else.
"Damn you, Nightmare, answer me! Please!"
The grotesquely deformed pony could not possibly have been keeping up with her now, especially given the fact that when she saw it, it had been dragging itself across the ground. Nevertheless, she continued running, glancing backwards, and repeatedly casting her light spell just to be sure that it was indeed not close by.
"I'm sorry, Nightmare Moon, alright?" Luna said, hating herself for doing so. "I'm begging you, please, speak!"
Silence. That horrible, uninterrupted silence. Even the disfigured pony's moaning had ceased.
Finally, Luna decided to stop running and skidded to a halt, gasping and trying to catch her breath.
"What did it say to me?" She thought out loud, half-expecting some snide remark from Nightmare Moon but unfortunately receiving none. "I was glowing...glowing with..."
"Life, my dear." A clear, well articulated, stallion's voice replied, before chuckling. "You can't see it, can you? I haven't had a live one down here in ages..."
That voice...it couldn't possibly...
"You're Luna, correct? I never forget a face, after all." The stallion spoke again, and Luna knew she had heard his voice before. "I'm truly sorry for that pony back there, by the way."
The stallion once again laughed, and Luna could hear the sound of magic being cast. In seconds, she could feel the tingle of magic enveloping her, its aura creeping through her hide. Suddenly and inexplicably, Luna felt herself being teleported and promptly blinded as she went from total darkness into a well-lit room.
"Now, no need to be shy, Luna." The stallion assured. Luna squinted in the light as she tried to make out who he was, but after so long in total darkness even he was little more than a blob of color. Slowly but surely his features came into view; a horn, a crown, a flowing black mane, a red cape...
"Would you like some tea, Luna?" King Sombra inquired politely, motioning towards a kettle which he floated over with magic. Luna instantly keeled over and found herself stumbling backwards, knocking over a glass table which hit the marble floor and shattered.
"YOU?!" She bellowed, blinking in disbelief. She had somewhat expected to have run into him here, but certainly not like this! And...was she in...her own castle? Indeed, it looked very much like her own, but with a distinctly...Sombra-esque design choice; the overall result was a terrifying blend of Celestia's eye for royalty and Sombra's eye for death and destruction. Everything was dark colors, and blue torches hung on nearly every wall, alongside vivid tapestries depicting night skies, the Crystal Empire as it appeared under his rule, and even some place in Tartarus.
"Of course, me!" King Sombra said, extending a hoof which Luna stared at quizzically before realizing it was intended to be shook. Sombra shook his head sadly when his greeting went unacknowledged, before theatrically parting the tapestries to reveal a twilit, mirrored version of Luna's own kingdom.
"Welcome, Princess Luna of Equestria, to my Kingdom! Welcome to the Underworld!"