Twilight couldn’t say for sure what had finalized her decision. Maybe it was continuing to adjust to life on the throne. Maybe it was the romantic (and slightly lava-crisped) atmosphere of Pinkie and Cheese’s Haywaiian wedding. Maybe it was bonding over a mutual appreciation for the romantic novels of Ducky Ink. Whatever the case, by the time the royal chariot carrying her and her friends landed in Ponyville, she knew what she had to do.
As everypony disembarked, she held back one mare in particular. “Rarity? I have something I need to say to you.”
Rarity paused and looked up at her, fear and hope mingling in her eyes. “Yes, Twilight?”
“Even since I took the throne in Canterlot, there’s been something missing from my life. Something I had taken for granted while living here in Ponyville. More than anything, I want that something back. The friend who took plain, ordinary Twilight Sparkle and made her feel like somepony special long before she got her wings. The one who’ll listen to me describe the latest discovery in arcane physics and then return the favor with the new fashions in the upcoming season. The one who carved out an empire long before I inherited a kingdom."
The reigning princess of Equestria knelt. “Rarity Imogene Belle.”
“Yes?” said Rarity, eyes wide.
“Will you…”
“Yes?”
“… come with me to examine the Helix of Hof next week?”
“Yes, yes, a thousand times…" Rarity's face fell mid-bounce as she finally registered the specific words. "What?”
"Well, I am going to make a diplomatic visit to Saddle Arabia," Twilight said with a shrug of her wings. "May as well take the opportunity to investigate a local landmark."
"Ah. Well. I see." Rarity visibly drooped with every word.
Twilight winked. “And I’ll need the time to find somepony other than you who could make an engagement ring worthy of that horn.”
That got her a smirk and a swat on the wither. “Twilight Sparkle, you are absolutely incorrigible. Very well, it's a date." Rarity narrowed her eyes. "Though I do want to be sure, is there any chance of some manner of magical nonsense intruding on what I hope will be a perfectly romantic moment in an exotic locale?”
Twilight shook her head. “None whatsoever.”
“Okay, so I may have underestimated the possibility of magical nonsense.”
Rarity gave Twilight a flat look as scraps of tangible shadow swirled around their shield bubble, shrieking in voices like tortured metal. “You don’t say?”
Twilight winced more from the scathing tone than she had from any of the claws of semisolid darkness scraping against the barrier. “In my defense, there were no indications of this sort of thing.”
“None whatsoever?” said Rarity, looking across the windswept desert. The Helix of Hof lay in the center of a cracked, gray waste that made the surrounding sands seem as lively as Fluttershy's backyard. There wasn't a cloud in the sky, yet the sunlight still felt wan and washed out by the time it reached them. The Helix itself was made from almost organic looking obsidian, thick as a pony's barrel and coiling in a circle as wide as five ponies standing nose to tail. Each loop seemed more horrific than the one below, bubbling, distending, and splitting as though it were diseased.
Suffice to say, not the most picturesque location in Saddle Arabia. And that was before setting hoof in that gray wasteland had set vengeful, unliving shadows swarming around them.
“Okay, so there may have been vague legends about some kind of marauding force from outside our universe creating the structure to more easily feast on our reality, but I couldn’t substantiate any of them!" Twilight threw her hooves into the air. "Most weren’t even peer reviewed!”
Rarity took a deep breath. “Twilight. Dearest. I adore every inch and aspect of you, but I could say the same about Sweetie Belle. And I can honestly say that no amount of molten toast will ever compare to this.”
Twilight sighed. “Yeah, that’s fair. At least the ring's nice?”
That got a brief smile out of Rarity, at least until another tormented soul wailed like a dying anvil. Strands of platinum beautifully woven around three briolette-cut sapphires could see her through a lot, but this seemed to be her limit. "It is, but as far as accessories go, I'd rather have the Element of Generosity right now."
"Again, fair."
“So, how exactly do we get out of this?”
Twilight's expression shifted to one of utmost concentration, the sort Rarity only saw when Twilight faced either a threat to Equestria or a new library sorting method. “Well, the Helix of Hof is a single immense thaumic resonator. That’s why I brought you here; it’s the sort of object only a unicorn can fully appreciate.”
“Lovely sentiment, darling, but please get to the point.” Rarity chose not to say anything about how the appreciation apparently took the form of feeling one's horn get dipped in acid whenever one used magic.
“Right, sorry." Twilight nodded at the swarming shadows. "These creatures are definitely tied to it somehow, malice incarnate perpetuated by the energetic feedback. They’re just not substantial enough to exist without some kind of keystone or linchpin to sustain them.”
Rarity nodded along. “So we just need to find that one seam that holds them together and tear it out.”
“Right." Twilight eyed the spiraling structure nervously. "Ideally while preserving the Helix.”
“You’re vastly more important than some twisty little rock formation,” Rarity said with a click of the tongue.
“We’re on foreign soil, and if we destroy the local landmark, we may end up at war with Saddle Arabia.”
That took Rarity a moment to process. “You’re still more important than the twisty rock formation, but I acknowledge that it is important in its own right.”
“Great." Twilight limbered up, making Rarity thoroughly aware of how much she'd grown, even after less than two moons on the throne. "So, I can distract the shades. I need you to probe the Helix with your gem detection spell. There must be some central crystal that’s the focus of all of this negative energy."
Rarity nodded, eyes focused on the Helix and definitely not anypony's earth magic-enhanced muscles. “Say no more.”
Twilight bit her lip. “Uh, are you sure? Because the next bit isn’t exactly intuitive.”
“I acknowledge that I don’t have quite your level of expertise, dear, but I think I’ve gone through adventures to know how to disable a little ancient relic.” Rarity smiled and hopped up, pecking Twilight right on the mouth. "For luck."
Even in their situation, Twilight gave a goofy grin. “R-right. Good luck!”
“It was for you, dearest. I already have all the luck I need, and she'll be watching out for me.”
If anycreature asked, Rarity charged towards the Helix with a bloodcurdling battle cry worthy of the warrior ancestors she probably had somewhere in her family tree.
It was a very high-pitched battle cry accompanied by desperately trying to keep shades out of her mane, but a battle cry nonetheless.
As she approached the Helix, the ground became less sand and more heavily compacted dust, cracking and powdering under her hooves. She paid it little mind. It barely showed up against her coat anyway.
Rarity ducked under the lowest obsidian loop. Within the Helix, the ground was bare stone, still dreadfully gray, but much more solid footing. Twin trails of black glass drew a diameter across it, and at the center they coiled together into a small, spiral peak that ended at her eye level. There sat a chicken egg-sized opal that swallowed what little light made it in the structure, glimmering as malevolently as the heart of the Alicorn Amulet.
"Well. I suppose... this is... the simple part." Rarity walked to the dais as she caught her breath. She lit her horn for a moment, then gasped and released her magic before she could even form a spell. If the local magic had burned like acid at the edge of the gray, here it was more like setting her horn on actual fire. "Or not."
The light went purple for a moment, Rarity's shadow stretching out before her. She shook herself, refocusing. "For Twilight." She reared up, pressed the black opal between her forehooves and pulled.
Absolutely nothing happened.
"Come on! Come on!" No amount of tugging or yelling helped. "Oh, what do you want?"
The answer hit her mind without any crude intermediaries like speech. She could not take here. There had to be an exchange.
"Ah." Rarity grinned despite herself. "Well, you have the right mare for the job there."
Twilight panted, wings drooping and horn guttering. Even she could only go on for so long, but as long as the Helix was functional, the shades had functionally infinite energy.
But, just as she saw one lunge in from her blind spot far too late to do anything about it, they all froze up. Wisps of darkness evaporated off of them, and the full desert sun struck the area. The shades burst into unearthly blue flame, and soon were no more.
“Twilight!" Rarity cried, galloping towards her from the Helix. "I did it!”
“I know!" Twilight raced towards her, but slowed to a trot as she thought. "Though... No offense, but how did you know how to propagate the fractal hairline crack that would disrupt the energy flow?”
Rarity blinked at her. “Well, ah, I didn't."
"You didn't?" Twilight shivered despite the building heat.
"I just removed the large, black opal from the middle and put something in its place.” Only then did Twilight notice Rarity's hornring was missing. And that she held an enormous gem. "I thought the power of true love would help dissipate those ghastly beasts."
Twilight's jaw dropped as she contemplated the ramifications.
Rarity pouted, fluttering her lashes. “Is that a problem, dear?”
After a few moments and swallowing against the lump in her stomach, Twilight got out, “It might not have been one if you hadn't had so many other brushes with dark magic.”
“Ah." Rarity contemplated the opal, and the way black oozed out of it into the magic holding it up like ink in water. "Oh dear.”
A beam of negative radiance blasted her, consuming her in a cocoon of antilight before revealing a huge, black-coated, and dreadfully familiar mare.
“Rarity?" Twilight bit her lip, trying not to add "Nightmare" to the name. "Please tell me I don’t have to call in the human girls to harmonize you. I’m pretty sure Sunset Shimmer still hasn’t forgiven me for not inviting her to my coronation.”
The mare looked herself over and smiled. “Honestly, I feel fine." The smile turned to Twilight. Rarity chuckled. "And I do appreciate the chance to look you in the eye again.”
“We’ll still get this looked at.”
Rarity patted Twilight on the withers. “Of course, dear. But after the wedding.”