The skies were dotted with the colored smoke flares of celebration and the more somber black flares of mourning as Equestria saluted both the salvation of their nation and the valiant sacrifice of their king, and Princess Luna took care not to disturb any of them as she made her way out of the populated areas.
Less than a week ago, perhaps a few brave souls might have foolishly attempted to send some magical volleys into the sky after her, but Sombra’s council worked very quickly. Already the official story had been spread from border to border that she and her sister had been possessed by a malevolent and ancient magical entity, one that had only been purged when Sombra had drawn it from them and into himself. It had just enough truth in it to hold weight, and ponies had been remarkably willing to forgive.
But, frankly, what the populace thought of her mattered little at the moment. She had much more concerning things on her mind.
Once reaching Everfree, Luna touched down, only briefly stretching her aching wings before she took a deep breath and leaped off the cliff and back into the clouds. It had been ages since she’d had to spend so long in the air, but she couldn’t afford to stop yet.
====
One wingbeat, then two, then three, then she broke through the cloud bank and consulted the magically-blessed compass and map pendant dangling from a delicate silver chain around her neck for the fifth time in as many hours. Once upon a time, it had been a birthday gift from Starswirl and the other world’s Celestia; one of the many little amusing trinkets she’d abandoned in Canterlot castle when her sister’s black magic influence and manipulatively sweet words had taken hold. Now, it was her only method of keeping track of Sombra, who’d proven as adept at hiding his magical signature as he was with shields.
"Where are you going?" she muttered to herself as the tiny pinpoint of blue-turned-black that marked his progress changed direction once again, heading north. She had been following its seemingly aimless path for over a day, almost catching up to him before he would suddenly veer on another course. Trying to teleport directly to him had been useless, as without an ability to directly lock her senses to his location, she’d repeatedly ended up missing her mark.
Luna watched the little light for several more seconds until she was satisfied that he was going to be staying on this northern path for at least a little while, then tipped her wings and swooped off to continue the pursuit.
===
Two days of trailing the signal later, she landed in drifting snow to find that the compass and map had guided her to a cave in the north-facing side of the mountain ridge that formed Equestria’s northernmost border. It was seemingly nothing more than a blackened crack in the earth, too small and uneven for a grown, or even a teenage, dragon. But another check of the compass and map indicated that this was the correct place, so Luna lifted a wing to guard her face against the wind and snow and cautiously approached. “Som-“
The base of her horn itched in warning and she dodged out of the way of the dark magic that boiled out of the crack to strike the ground where she’d been standing, melting the snow and scorching the earth underneath black. She flared her wings defensively and braced herself as the shadows within the cave moved and separated themselves from the crack.
The creature that emerged from them, shedding them like a second skin, could no longer be called Equestria’s noble king. Green and red eyes glowed malevolently with black magic. Teeth bared in a snarl had sharpened into something more dragon-like than equine. Blue-grey hair had darkened to the same black as the shadows that had fused with him, and his coat had taken on a strange, dark, oily sheen. And the stance he’d taken as he glared hatefully at her could only be labelled feral, as if he were waiting for an opening to pounce and tear her to pieces with those unnatural fangs.
Luna swallowed thickly, but refused to back down, though fear prickled all the way from between her ears to the root of her tail at this disturbing, alien creature that had once been her closest friend. Straightening her back, she raised her head defiantly and spoke as calmly as she could manage. “Sombra. It’s been a long trip to find you.”
She wasn’t sure what she was expecting to happen but, surprisingly, he shuddered at her voice, gritting his teeth and shaking his head several times as he took a step back.
“Lu-na.” The sound of her name in his mouth was too deep, with too much growl, but it was him. “Luna? You aren’t supposed to be here. You should go.”
"You-"
“Leave!" he snapped, and that was the only warning she got before another burst of dark power erupted at her; she took to the air to avoid it, and when her vision had cleared, he was gone.
Back into the cave, most likely.
She clenched her jaw as she landed, throwing caution into the snowy wind. A flare of sparks at her horn summoned up a protective shielding over her body and she conjured up a torch spell to go with it before bolting into the cave after him-
-only to skid to a halt at the sight that met her.
Crystal.
The walls, the floor, the ceiling… all were made of crystal. And with such a small opening to the outside world-
Suddenly, the winding paths he’d been taking made sense. He hadn’t been trying to evade anyone, he’d been searching for-
She ducked as a tendril of shadow lashed out, whiffing past her face so closely that it penetrated through her mane before withdrawing into the ball of dark magic that slowly reformed itself into the unicorn king. “I told you to get out,” he snarled, voice barely retaining words over the guttural roar that had nearly claimed his throat again.
"I won’t! Not while you’re looking for a way to die!” Luna snapped in return with a stomp of her hoof.
That seemed to briefly startle him back to himself, as he backed off again, looking away with what almost seemed like shame on his face. “So you found me out.”
"Look," Luna said gently, carefully moving forward. "You don’t have to do anything so drastic. Just come back to Canterlot and we can find a way to fix this-"
“No!" She dodged another tendril at the snap, and Sombra turned back towards her. The malevolence in his eyes had returned, but it was tempered with a mixture of fear and desperation. "I don’t have that long! The harder I fight, the more it destroys! Even you can’t see how much it’s changed me already!” He advanced closer, and it was all Luna could do to hold her ground and not give in to the instinctive urge to back up very quickly. “Even now, as hard as I’m trying to keep control, my brain is filled with a dozen different ways to kill you. And that’s only the beginning. I see visions of thousands of other atrocities, and myself laughing at them! This has to end now, before I can’t keep myself from trying to act on them anymore. Before another war begins, or something worse!”
As nauseating as it was; as much as she didn’t want to believe him, there was unfortunate truth in his words. Even as hard as he was struggling, Luna could see her own death in his eyes, the seconds ticking by bringing a more creative demise than the last. It made the hair and fine down between her wings stand on end. Had she been like this when she had given in? Was this what it had been like for him when she’d turned on him for her sister?
Her sister…
The memory of the other Celestia reminded her why she’d followed him all this way to begin with, and gave her a new idea. “Then let me help.”
Thrown off by her words, he merely blinked at her. “What?”
"Let me- let me do the sealing. Yes, sealing,” she reiterated when Sombra gave her a disbelieving look. “The same as what you and the Element bearers had originally planned, not…this.”
"It won’t work," Sombra said, voice labored as the shadows around him began to writhe again. "It’s not- the darkness isn’t the same when it’s contained in just one body instead of two; you can see that already, it-“
"It will work,” Luna insisted. “Because I can put you to sleep first. Even if the darkness can reach your dreams, so can I. Then we’ll both be able to fight it.” He shook his head, but was visibly straining against the black magic’s control, legs shaking and buckling under the stress. Taking a chance, Luna moved in close, then reared up and wrapped her forelegs around his neck. “She wouldn’t want it to end like this. For you to suffer and kill yourself like this. I promised her I’d take care of you, and that’s what I’m going to do.”
At the mention of his beloved Celestia, Sombra finally made a hitched noise that sounded like a mix of a snarl and a sob. “Do it,” he gasped, then shoved her away as the shadows surged up in an attempt to grab her. “Hurry!”
Luna flapped away from the grasping shadows, reaching out with her magic to blast crystals off the walls and columns and fling the sharp projectiles in the path of her pursuers. Gathering speed, she circled back around and pooled magic into her horn for the strongest sleeping spell she could conjure-
-and just as a tendril snagged her left hind leg, she fired.
===
Staring at him lying on the floor was uncomfortable. Though she knew for certain that the spell was only for stasis, not killing, seeing him so still and not breathing made her nervous and she had to kneel down and press her ear to his chest, just to be sure that he still had a pulse at all. Relieved to hear the soft, sluggish thump-thump, she sat up, trying to gather her scattered thoughts back in order.
Her first thought was to bring him back to Canterlot. But centuries of magical warfare had ravaged the crystal caverns beneath the city; finding enough raw material that hadn’t been tainted by her sister or herself or those loyal to them would be next to impossible. No, to keep him safe, it was best that he stay here, where only she knew his location and there was plenty of unscarred crystal to be had.
Decision made, she settled herself in for the long haul, breathed deeply for several seconds, and set to work.
Clear crystal was magically manipulated until it became liquid, and then drawn up beneath him in a pedestal. She carefully arranged him into a noble, yet relaxed position, then formed the crystal around him in a protective shield before building it up into the cave ceiling, creating a column.
Cracking an eye open despite her concentration, she couldn’t help just the tiniest little grin of pride at how flawless it looked. Sombra had always taken to Starswirl and the other Celestia’s magic lessons a lot quicker than she had, especially where the manipulation of crystal was concerned. She wondered if… maybe if things had gone differently… they would have been proud to see that she’d finally gotten the hang of the more complicated spells. Thinking about it made her heart hurt and her focus falter, however, so she quickly shoved that aside and put her mind back on her work.
More clear crystal was drawn up the column, slowly building out from the center to form a ball around its occupant. She squeezed her eyes shut so tightly they hurt in an effort to both keep it perfect, without cracks or air bubbles, and to infuse it with shielding magic at a strength no unicorn could match.
By the time she was sure it was thick enough, her vision was swimming, her skin was bathed in sweat and she was quivering. She heaved out a pained groan and, not caring that she was getting her veil wet, flopped into a small drift of snow that had blown in from outside and wriggled a bit to cool herself off.
When she’d finished, Luna walked up to the column and gently pressed a hoof against it as she scrutinized her work. Good and solid and well, protected… but still... lacking. It needed… something. Looking around the cavern at all the colorful crystals that lay scattered about in spears and shards, she had a feeling she knew what that something was. A quick burst pulse of pressure sorcery caused more to shatter free without disturbing any load-bearing columns, and she swiftly got busy.
Blue, Purple, Red, Pink, Orange. She sheared the pieces into glass-thin panes and carefully molded them into the central ball in a sweeping, looping sunset-like pattern that would have made the other Celestia proud, with more wards and protective spells etched in tiny hidden runes in each piece. Once the entire globe had been covered in the stained-glass mosaic, she stepped back and lowered her horn, etching blackened words into the base of the crystal column.
A Hero of Equestria I lie
My long Sleep well-deserved
O fair Traveler passing by
Please let me rest undisturbed
Princess Luna studied the words, then wrinkled her nose. A poet she was not, but it would do. The globe and its column would serve as a fitting ‘memorial’ for anyone who might use the cave for shelter or otherwise merely be curious, and for those who might wish to cause trouble, only somepony with an alicorn’s strength and knowledge taught by Starswirl would be able to unmake the barriers.
Finally satisfied, she looked up at Sombra’s still form. “See you soon, my friend,” she murmured, then turned and made her way back out into the snow.