Thirty-six hours later, Shining Armor flopped his digestive-juice covered body onto the newly-reestablished rope bridge spanning the Chasm of Harmony that encircled the Castle of the Two Sisters. A timely application of a self-targeted force shell had made him (suddenly) too big a morsel to pass through the digestive tract of the hunting cragodile that had briefly succeeded in eating him shortly before the timberwolf attack. Even now, he could hear the distant baying of the terrifying half-vegetable predators. Shining hauled himself forward across the planks of the bridge, leaving a trail of sludge and offal from the cragodile's gut. This'll deter the timberwolves, he thought, frantically. They're too smart to wander onto unhallowed country haunted by the leftover remnants of Nightmare Moon. Shining warily regarded the darkening sky. It had been his original intent to have all his business in the Everfree sorted out by nightfall, but the constant carnivorous plant attacks and the half-petrified left hind leg (cockatrice) had slowed things down a touch.
He shifted his gaze to the crumbling castle before him, raised his eyes to the tallest intact tower, and beheld a light in the darkness of his own personal night. It shone, gray and flickering, from the high observatory tower. Gathering himself, he clumped across the bridge, half-dragging his stone rear hoof, and crossed the castle's threshold to its echoing, immaculate Great Hall. Shining took a moment to look around the space and, once again, admired the hoofwork of his sister and her five friends. The old castle had been fixed up and polished into something that once again resembled a royal living space.
The whole business had been done in honor of Princess Luna, of course, who now maintained her permanent residence in Two Sisters instead of Celestia's Canterlot to keep the Diarchy's inter-sibling spats to a minimum. Obviously that's why it had been done. After all, why would anypony devote a lot of time and effort to cleaning and fixing up the old castle in Everfree if they were just going to leave it fallow, untouched, and completely unused, like some sort of abandoned plot point? That would just be silly. Obviously nopony would do that.
Wending his way through the echoing hall, Shining eventually located a grand staircase leading to the mezzanine, and bit by bit he eventually made his way to the far more narrow, far more winding set of steps leading to the tower. After a substantial climb, he found his way blocked by a stern wooden door with gray light seeping out the edges. Shining took a deep breath, summoned his courage, and gave the door a series of light hooftaps.
"Hark! A visitor!" came an overwhelming voice from beyond the door. Such was the force of the outburst that the door itself flew open and clocked Shining in the muzzle, knocking him ten steps backward and causing him to fall in a heap of limbs at the next highest landing. "Who sets hoof into the demesne of Luna, Princess of the Night?"
"It's... uh, it's me, Highness," said Shining, eventually managing to untangle himself, which was no mean feat given his half-stone leg.
"Oh, Shining Armor," said Luna, at a far more reasonable volume.
"Yes, ma'am," said Shining, clomping his way back up the stairs. "You sent for m—"
"Enter and be welcome, most exalted Prince of the Crystal Empire!" bellowed Luna, knocking Shining back down the stairs.
"Ow," he said, weakly. Despite being even more injured than before, Shining made the quick threat assessment that Luna was probably less dangerous close up, where she was less likely to unleash her devastating sonic attack. He clambered up the steps. "Yes! Coming!"
"Do not tarry, Shining Armor. My nightly duties come apace!"
With tarrying the last thing on his mind, Shining re-achieved the top of the steps and peered into the observatory dome. The sight inside was, well, a bit informal, especially when he compared it to Celestia's pristine and awe-inspiring public audience hall. Sprawled across a beanbag chair was Her Royal Highness Princess Luna, telekinetically eating a bowl of cereal and idly manipulating a vertically-mounted stick with her left hoof. The cereal, Shining saw on closer inspection, was not really cereal at all, but rather an entire bowl of only marshmallows that had been plucked out of the mix of a normal box of Magic Charms (tm). Shining noted with bemusement that, in addition to the actual cereal, all the pink heart, blue diamond, green clover and purple horseshoe marshmallows had been discarded as well, leaving an entire bowl filled only with yellow moons. Luna's stick appeared to be controlling the motion of a small white line of pure arcane energy in some sort of cavernous dark box. Luna's beanbag had been placed directly before the box, and it had her full attention.
Shining was just on the point of becoming confused as to the function of the obstructive thing when there came a pinging sound, and a tiny cube of white sorcery materialized within the darkened box. "Approach our royal throne, good prince!" Luna thundered, her eyes never leaving the tiny white cube. It seemed to drift in Luna's direction, but when it got close, Luna used the joystick to maneuver the little white line to intercept it and send it drifting back across the length of the box. At the far end, it encountered another line which moved independently of Luna's stick. With a sharp pinging noise, the second line sent the cube drifting back towards Luna.
Staring at the odd thing, Shining found his formerly-pressing concerns about Cadance momentarily evaporating. "Your Highness," he said, "what, um, in Equestria is that?"
"'Tis a marvel, no?" said Luna, looking over her shoulder. "We call it 'Ping.' We have spent many moons perfecting it. The goal is to use this controller to secure my side of the box against the wicked depredations of the white square."
"So, don't let it touch your side?"
"Precisely!"
"Like it's doing now?"
Luna's eyes went wide. "Gadzooks!" she cried, leaping at her control stick as though it were a departing train. it was too late; the white square crossed Luna's side of the line. An obnoxious klaxon sounded, and a floating white number "1" appeared on the far side of the box.
Luna's eyes went wide. She sucked in a deep breath. Shining looked frantically around for something to hold on to; unfortunately, a moment too late.
"Noooooooooooooooooooo!" cried Luna unto the heavens, or at least the portion of them visible through the observatory roof. The force of the utterance was such that Shining was flung back out the door and halfway down the stairs again. He landed with a dull thud.
"...ow..." repeated Shining Armor, weakly.
"Oh well, on to business!" said Luna, from above. "Where have you gone to, Shining Armor? We distinctly recall instructing you not to tarry!"
"Yes, Your Highness," said Shining, unfolding himself with some effort and thudding up the steps again.
"Make haste, and also, prithee tell us what business come you on this evening?"
"You, um, told me to come see you. Last night. In a dream."
"Oh, of course we did," said Luna, levitating a spoon full of milk and marshmallows up to her mouth. "Our apologies. The Dreamlands often do not follow the normal courses of time, space and memory. From our perspective, the conversation you speak of happened long ago. We scarce can even recall the details."
"I was dreaming Celestia was in my bed and then you blasted her in the face."
Luna spat marshmallow milk all over the floor. It was about half a minute before she could speak due to the debilitating sniggering.
"We remember that now!" she exclaimed, eventually. "That wast basically the best!"
"Ma'am, can we please—"
"Yes, of course," said Luna, wiping away a tear with her silver-shod hoof. "We were discussing alicorn pubescence, were we not?"
"Yes. Specifically, how to stop it."
"Oh, of course," said Luna, turning back to her game. "It is no use, Shining Armor. You cannot stop it."
"There must be something. I'm thinking about spending the rest of my natural life married to Her Royal Highness Princess Celestia and I'm not liking it."
"Be at ease, good prince," said Luna. "'Unstoppable' is not the same as 'immutable.'"
"How do we, ah, mute it, then? Tame it down?"
"It cannot be tamed. Only redirected." Luna set down her cereal, arose from her beanbag, and began pacing across the length of the room. "A mature alicorn," she said, "exists in one of two different states. One is that of a relatively normal, well-adjusted immortal goddess-pony."
"And the other?"
"Celestia," said Luna.
"I want the first one," said Shining. "Honestly, I think Cadance would want it too."
"Of course you would. Thankfully, we have a solution. We are in possession of an ancient, mystical scroll of great magnitude, potent physic against matters of misaligned destiny." As Shining watched, Luna strode purposefully over to a coffee table and began searching through the layer of assorted detritus that topped it. "Where didst that dumb thing go?" she muttered to herself.
Shining waited patiently, afraid of engaging the Moon Princess in unnecessary and potentially-hazardous conversation. Eventually, Luna dislodged a timeworn ebony tube from the mix. It tumbled to the floor. "Aha!" she cried. "Starswirl the Bearded's second unfinished spell!"
"...Second...?"
"Yes. Know'st thou how the first one made a hash of the destinies of normal ponies?"
"My sister composed this whole big song about it, so yes."
"Well, this one makes a hash of the destinies of alicorns."
"That sounds... titanically unsafe, ma'am."
"Pah," said Luna. "Only in the hooves of somepony who isn't the princess of all friendship, and thus reflexively, magic."
Shining winced. "Twiley will never go for it. Not in a million moons."
"We have confidence you will win her over," Luna replied. "But before we begin the fraught process of tinkering with destiny, are you certain your spouse is headed down the dark path to Celestiahood?"
Shining thought hard.
* * *
"It's great to have you and Cadance over to visit again!" Twilight gushed. "Plus, the adorable presence of li'l Flurry Heart gives me time to break in my new castle's Crystal Nursery!"
"You have, um, a nursery?
"Yep! Only the most comforting and nurturing jagged friendship-crystal edges. I'm telling you, B.B.B.F.F., this new castle has everything." She leaned in close. "Everything."
"Er, great," Shining said as he approached the in-castle replica of his foalhood room, A thing which had been exciting at first but which was gradually getting weirder and weirder the more he thought about it.
"Cadence has been up here unpacking," said Twilight, pushing open the door. "She's really been into that old ant farm of yours!"
Twilight pushed the door open. Shining Armor, wincing, peered in. He found Cadance sitting in front of the shelf with the ant farm on it, gazing at the little box of sand with an expression that might charitably be described as "unhinged."
"Oh, hey, little guys!" said a transfixed Cadence, her eyes wide and her face fixed in a wild grin. "Just look at you scurrying about your little lives down there. It all looks so orderly and peaceful!"
She lifted the box in her telekinetic aura and gave it one good, strong shake.
"Ha!" she shouted. "I bet that'll give you all kinds of opportunities to learn!"
"Honey..." said Shining Armor.
"I EXPECT LETTERS!" she screamed, at the box.
* * *
"Absolutely no doubt about it," said Shining Armor.
"Well, then. We entrust you with this most sacred and dangerous spell." She floated the scroll over to his teleactive radius, and he gingerly took hold of it. "Use it carefully, for the health and wellness of your alicorn bride."
"Don't worry, ma'am," said Shining Armor, saluting. "Between me and my sister, this scroll is in good hooves." His face darkened. "I still have no idea how I'm going to convince her to help me, though."
"Fear not, good prince," said Luna. "We know of a way."