Luna wasted no time in dashing towards the towering glass window to behold the sight of Equestria. Before she even had a chance to look out, however, she was greeted to the surprising sight of herself reflected in the glass. There she was, looking every bit the same as she did before she had transformed into the dark beast that killed Celestia. For a long while, she simply stared into her own eyes, taking in the sorrow she saw glaring into her own soul.
Shaking her head and focusing beyond the glass, she saw she was looking at her own world, lit by an odd crepuscular half light much different from what she had come to know as 'twilight.' Furthermore, the presence of it seemed impossible, considering the complete lack of anything resembling a sun or moon that could possibly give off any light in the first place. Similarly, it looked like it was raining, but again this seemed logically impossible given the lack of clouds.
There was something else about Equestria that was bothering her, too. For whatever reason, everything seemed flipped; as if she was looking at her own familiar world, with all the land all the same in appearance, but reflected through a mirror.
"Tell me, Luna." Sombra said, walking towards the window until he was standing next to Luna. "What do you see when you look out across the Underworld?"
"Equestria." She answered without a moment's hesitation.
"Fascinating. Myself, I see something completely different. I see the Crystal Empire as it was when I was king. I find it fascinating that you would see your world exactly as you remember it."
"Well, structurally it looks the same." Luna admitted. "But it looks like a mirror image or something."
"Of course it does. Most ponies make the mistake of assuming the Underworld is one literal, concrete place where everything winds up when it dies. The reality, however, is that everypony's Underworld is different. Yours is apparently a reflection of your former kingdom."
The word 'former' took Luna by surprise, and she opened her mouth to object immediately, before realizing that Sombra was in fact correct. She wouldn't be returning home, and therefore had no authority calling it hers.
"Of course, nopony would actually know any better, because upon trying to enter the Underworld using dark magic, they wind up in that glorious little limbo realm that you just left."
"Huh." Luna muttered simply. She had planned for this to have been a straight forward task, but apparently this was going to be harder than she had thought. "Sombra, I'm down here for a reason. Perhaps you can help..."
"Yes, I would assume you would have a reason for coming down here." Sombra said, amusement in his voice. "And of course, it's my duty to help you."
"Really?" Luna rose an eyebrow skeptically. "You carry no grudge against me because of my—"
"Luna, Luna, Luna. How petty do you think I am? My death is ancient history, probably quite literally by now. Besides, I've gone from being a tyrant king to a God over the dead. So, in all honesty, thank you."
"Well, I'm..." Luna was taken aback, but managed to maintain her solemnity. "I'm happy for you."
"Yes, yes. So then, your reason for visiting me."
"I need to ask a favor of you. Granted, I wasn't expecting you to be the one to grant it, but no matter. I need to ask you to raise somepony from the dead."
"Oh my, one of those favors, I see." Sombra instantly took on a very serious tone. "You always were fond of dark magic, weren't you, Princess? Raising the dead is no light matter."
"I know it's possible. Do not even try to fool me, Sombra."
"It's not that." Sombra sighed, signalling towards one of the towering murals on the cold brick wall of the throne room, depicting Tartarus itself. "You're right, it is possible. Actually, surprisingly simple. But not for me. I may rule the Underworld, but it is the Lord of Tartarus who decides the fates of the dead."
"Are the Underworld and Tartarus not synonymous?" Luna said, casting a disturbed glance at the gruesome painting of the cursed realm of the damned.
"Certainly not! Tartarus is a different realm ruled by a more powerful being than myself."
"Who?" Luna inquired, although she already suspected the answer.
"I wouldn't know. And I would just as soon never want to find out." Sombra said, motioning upwards at the painting to prove his point. Why he even kept it in his throne room was beyond Luna's comprehension. "I care about my realm, and he cares about his."
Luna let out an audible groan of frustration and rolled her eyes. This was a set back she had not been anticipating, let alone prepared to deal with.
"Well in that case, I need to get to Tartarus." She stated pounding the floor with her front hoof. "Is that not possible?"
"Of course it is! But no sane pony willingly enters Tartarus! It's a cursed realm from which return is prohibited!"
"I ask not for your judgement." Luna rolled her eyes again. "I ask for your directions. Tell me how I get there."
"Fine, fine. No need to get upset." Sombra cleared his throat. "Don't say I didn't warn you. Tartarus is one of several specific areas of the Underworld which remain consistent across every Underworld. My castle, for example, is another. While the entrance will vary from Underworld to Underworld, once inside it is the same place across all of them."
"Then where is it?" Luna snapped impatiently.
"I don't know! It's your Underworld!"
Luna was growing restless by the second as she paced back and forth across the windows. Sombra obviously wasn't helping her any more, and now that she knew her journey would not be as straight forward as she had planned, it was beginning to cast major doubt on whether or not she should embark on it in the first place.
"And Luna..." Sombra was saying, Luna only partially paying attention. "You do know the price for raising somepony from the dead, right?"
"I am willing to give up my own life for the pony in question."
"Good, good." Sombra muttered, deep in thought. "That is...brave, I suppose. Hopefully your opinion doesn't change when you see what Tartarus beho—"
"It shan't." Luna asserted, standing tall and flaring her wings. "If that is all, then I shall be setting out."
Giving the king a respectful nod and bow, Luna turned towards the throne room window, opening the glass with her magic and unfurling her wings.
"Wait!" Sombra cried, dashing after her. "Before you go, I should warn you about your companion."
"Nightmare Moon?" Luna asked, surprised Sombra knew of the voice only she could hear.
"Is that what she calls herself now? Yes, her."
"Do not fear." Luna assured. "I already hate her with every fiber of my being."
"That much is easy to say," Sombra argued. "Do NOT trust her. No matter what happens, you cannot trust her intentions."
"Noted." Luna said. She had figured all this out for herself, and Sombra's reminder truly did seem unnecessary, but nevertheless it was refreshing to receive confirmation that her dislike towards Nightmare Moon was justified.
Without further ado, Luna turned back to the window and took in a deep breath of the cool night air, before leaping out and taking flight over the twilit Underworld.
The first thing Luna noticed once she was outside was that she had been mistaken about the rain. What she had assumed were simply large, long streaks of rain turned out to be much thicker than what the window had betrayed. Furthermore, upon closer examination, she could see that they were in fact not falling to the ground, but instead rising from the earth. Indeed, it seemed as though thick, black streaks of pure darkness were slowly and gradually rising from the ground, before exploding and dissipating across the jet-black, starless sky above.
In addition to the reverse-rain, Luna observed an odd, otherworldly humming sound that seemed to be coming from everywhere and nowhere simultaneously. It sounded not unlike the sound one could achieve with their hooves on a wineglass, but much more ethereal and alien. Occasionally as she flew, she heard the eerily soothing humming raise or lower in pitch, without once changing in volume or developing into any sort of melody or rhythm.
Contrary to what she had been expecting, the Underworld was, in fact, populated. As Luna flew, she could clearly see signs of other ponies' presence; roads, houses, fences. On multiple occasions she even saw the warm glow of lantern light shining from one of the lonely houses along the road. She was intentionally flying low enough over the road that she could see if somepony was traveling along it, hoping to inquire where the gate to Tartarus could be found. She had intended not to make a large scene, making some lone pilgrim the ideal pony to ask. However, it seemed like while the Underworld was indeed populated, it was only scarcely. She would have liked to ask Sombra why this was, but she had a job to do.
All the while, Nightmare Moon did not once speak. Luna was beginning to wonder if the demon had finally left her altogether, when the tell-tale sound of wagon wheels thumping against the earth below grasped her attention instead.
Sure enough, Luna instantly caught sight of the wagon traveling along the dirt road, being pulled by a grey stallion with a matching grey mane. Another wagon was being pulled some distance behind it, but it looked like they were traveling together for safety as opposed to friendship; the front wagon that Luna intended to approach was purposely maintaining their position far ahead of it. Everything, from the decrepit, ramshackle wagon, to the faces of young colts and foals peering out from the shadows of the tattered interior filled Luna with sorrow. It looked like a traveling family, younglings and all. What had happened to steal this family's life and send them down here, Luna preferred not to know.
Their past aside, however, they were still an asset to Luna, and whatever information about Tartarus was welcome. Wishing not to alarm them, Luna made sure to fly overhead, before circling back around, landing on the road and nodding at the convoy which had ground to a halt.
"I bid you good...night, I suppose." Luna called after them. She had been hoping to avoid startling them, but it looked like not surprising the convoy had been an impossibility.
"My goodness. It's...an....an alicorn!" The stallion pulling the first wagon muttered, more to himself than anypony else. "How can that be?"
"I'm sorry!" Luna said, trotting forwards and giving him a reassuring smile. "I realize I must have startled you! I just need directions, then I'll be—"
Before Luna could even finish her sentence, the stallion broke into a sprint, nearly running her down as the wagon rumbled by. Along the way one of the wagons must have hit a fell area of the road, for it suddenly swerved and crashed onto the ground. The ponies within wasted no time in climbing out and fleeing down the road all the same. She called for them to wait and unfurled her wings to pursue them, but ultimately decided giving chase would just frighten them more. If they heard her pleading for them to stop, they did not listen.
"Wait!" Luna called after them hopelessly. "Please don't be afraid of me!"
Spitting a frustrated curse, Luna watched them disappear.
"Luna, Luna, Luna..." A patronizing and familiar voice laughed behind her. "Still making the same foolish mistakes, I see."
A sudden, blinding flash followed as she whipped around, and when it cleared away, Luna could see the distinct shape of a tall, slender black alicorn with sharp, fang-like teeth and a long, black shimmering mane towering over her. Nightmare Moon looked down at Luna, her mortifying expression slowly changing to one of apathy.
"Luna, when in Equestria are you going to learn?" Nightmare Moon hung her head and cackled, before offering Luna a hoof and helping her to her feet. "I don't think they were afraid of pretty little Princess Luna, after all."
"Nightmare Moon?! I...I can see you?"
"Thankfully, they could too. Once again, I assist your worthless hide."
"But how?" Luna gasped, poking her alter-ego with a hoof. "You're a voice! A product of insanity!"
"Never mind how. It doesn't matter. We need to talk." Nightmare Moon motioned around herself at the desolate landscape. "I'm sure even you know by now that this place is anything but normal."
"But why—"
"Shut up and listen, you idiot! You want to go through with this plan to bring back your sister? Fine. But if you want to stay alive long enough to see it through, you do as I say. Understand?"
"Yes, I do." Luna sighed in defeat. Her recent failures, both here and back in what Sombra had called 'limbo' really left her in no position to argue. Instead, she nodded several times and hung her head, saying nothing and simply staring at the road ahead.
"Why did you abandon me?" She whispered.
"Did I give you a scare?" Nightmare Moon snickered. "Good. I wanted you to get a proper feeling of...well, just how reliant you are on me."
"Thanks." Luna grumbled sarcastically, falling to the ground and staring up at the pitch black sky above.
"Well, we should get going." Nightmare Moon said, impatiently flaring her wings and shaking her mane, pointing her muzzle towards the south. "We have a sister to resurrect."
While Luna was taking a quick look around at the abandoned wagon for resources, Nightmare Moon took off into the sky with no explanation, leaving Luna with no other choice but to follow. The only thing of any use to her anyways was a sizable saddle-pack, which she fastened to her sides in mid-flight as she took off in pursuit of Nightmare Moon.
"Where are we going?" She called after her.
"South, stupid. Where else?"
"But what's south?"
"Tartarus, obviously."
"You know this how?" Luna said, gasping as she took in a large gulp of the cold night air. That was another thing about the Underworld she'd noticed; the cold. Despite all the bodies of water she'd seen, none of them were frozen over even though she was almost certain the temperature here was below freezing.
"I know because I'm not a fool, like you." Nightmare Moon scoffed. "But if you must know for certain, Tartarus is located in whatever part of the Underworld holds your deepest, most sincere regrets."
"The Everfree." Luna instinctively said.
"Bingo." Suddenly, Nightmare Moon exploded into purple mist, swirling around the air in front of Luna for a bit before disappearing completely.
"Not again." Luna groaned, watching her vanish.
"Oh, relax, Lulu. I'm here." Nightmare Moon let out another harsh sounding laugh, once again a mere voice in Luna's head. "Just because the Underworld gives me a physical form doesn't mean I have to use it all the time. Flying is tiresome."
"You seem to know a disturbing amount of information about this place." Luna mused, remembering Sombra's warnings about her companion.
"Do I detect a note of suspicion in your voice?"
"Perhaps you do. Could you perhaps explain?"
"Very well." Nightmare Moon sighed. Once again, the purple mist appeared, and Nightmare Moon manifested herself from it in direct reversal to her disappearance a few minutes prior.
"Remember back on the moon, when you asked me where my home was?" She asked, as she slowed her flying and looked straight into Luna's eyes instead of where she was going. She was frowning slightly, as if she were somewhat ashamed about something.
"Yes. You called me an—"
"Yeah, I know. I tend to do that. Sorry." Nightmare Moon exhaled deeply as she continued to glide, breaking her gaze at Luna to take in the sprawling plains of the Underworld-Equestria. "I was...withholding some information from you. The truth is, I do have a home. And it's here."
"The Underworld?" Luna rose an eyebrow. "What exactly are you?"
To her surprise, Nightmare Moon responded with outright hostility, something she had never done before.
"That, I have no reason to tell you." She snapped, barring her razor sharp fangs. "Sombra told you to distrust me, I'm sure. Well, go right ahead, but remember who saved your life, Princess."
With that, Nightmare Moon let out a contempt "humph" and once again disappeared in purple mist. Luna watched her go, not quite sure whether to feel anger or guilt. Directing her attention instead at the distant lands beyond, she strained to see the far-off reaches of the Everfree Forest. Perhaps if it were day, she'd have been able to see it, for it was not too far from Canterlot Mountain, but in the eternal twilight even the ground below was not quite clear.
She continued her flight for nearly an hour at a steady, calm pace, neither her nor Nightmare Moon speaking. Eventually, Luna broke her flight and began gliding towards the earth, landing beside a small pond in a wooded grove teeming with trees she had never before seen. They could have been anything from pine to birch, it looked to Luna like they were a freak parody of every single tree compiled into one.
"Taking a break?" Nightmare Moon said, once again appearing and taking a long breath of the night air.
"Yes, I suppose so."
"No objections from me. We're almost at Canterville anyway."
Luna perked an ear at the mention of a familiar town. This Equestria, albeit mirrored, should theoretically be the same, so technically it made sense for it to be present here, but it sounded as though Nightmare Moon was suggesting it was somewhere else.
"Hmm?" Nightmare Moon dipped a hoof in the pond and looked back at Luna, her eyes alight with amusement at the sight of Luna's curiosity. "Ah...Canterville isn't by the Everfree Forest, is it? My, it's so easy to get these two Equestrias mixed up. Well, let me tell you, Canterville is an adorable little settlement situated right beside the gates to Tartarus. We don't have to go there, but we should if you plan on kicking the bucket in eternal isolation, courtesy of your worst nightmares."
"I assume you mean Tartarus's punishment for me."
"Yeah, well, I'm just guessing." Nightmare Moon replied, slinking into the dark, black water of the pond. "Ah, it's so wonderful having a physical form like this!"
"Should I be afraid?" Luna muttered, fully aware of the improbability of a serious answer from Nightmare Moon.
"Of Tartarus?" She asked, dunking her head under the water and promptly resurfacing, wearing a playful grin. "I'm assuming you don't want me to mollify anything."
"Please." Luna said meekly, pacing the length of the pond. "Don't."
"Alright, Lulu, then I'll be straight with you. Tartarus is not a pretty place. If the Underworld is an eternal realm for the dead, then Tartarus is a prison for the damned."
"A prison?"
"Pretty much. You've probably heard horror stories about torture and eternal pain, but I assure you those reports are highly exaggerated."
"Will I have trouble crossing it, while I'm living I mean?"
"That, my dear Luna, is the most common misconception about Tartarus." Nightmare Moon yawned, wading back to the shore and shaking herself dry, not caring that she had just drenched Luna as she did so. "Being dead is not a prerequisite to being a resident of Tartarus. It's a prison for the evil. That being said. those who enter it living don't remain as such for long."
Luna shuddered as she recalled her confrontation with the mad stallion on the road. If ponies here were that desperate for life, she couldn't imagine what already violent ponies would be like.
"And the ruler? Sombra mentioned a 'Lord of Tartarus...'" Luna had several theories as to whom it might be, but absolutely nothing concrete.
"That...I would be lying if I said I knew the answer." Nightmare Moon confessed, sounding equally as frustrated by the Underworld's mysteries as Luna. "Reclusive fellow. Nopony's seen him. Likes to keep to himself."
That instantly shattered the most likely candidate for Luna's suspicions. She had wondered for great lengths where Discord would have fit in here, but assuming Nightmare Moon was telling the truth (which, it seemed like she was) she might not ever have to encounter him. She thought back to Sombra's assertions that everyone's Underworld was different, and decided that this was the most likely explanation for his elusiveness.
"Thank you for telling me this, Nightmare." Luna said without emotion.
"Yeah, whatever." Nightmare Moon shivered suddenly. "That water was cold. I don't recommend it."
After drying herself off, Nightmare Moon took to looking around her surroundings, her eyes finally resting on Luna, who was still staring at the pond with a broken expression.
"Why don't you get some sleep, Luna?" Nightmare Moon muttered, starting to place a supporting hoof on Luna's back but instantly backing away, as if the act of expressing compassion embarrassed her. "I can keep an eye out for trouble if you want to."
"Thank you. I think I will." Luna shook her head and put on a false smile. "Since when did you become so friendly?"
"Don't get used to it." Nightmare Moon blushed. "Shut up and go to sleep."