As Twilight walked towards the back of the Crystal Ballroom, her lit horn gradually dimmed. Was that the anomaly? Princess Celestia was quite vague as to what she was to investigate, only declaring that it was to be her top priority. Cadance was no more forthcoming. They didn’t want her to have any preconceived notions.

She tried compensating by upping her magical exertion. That worked. She turned around and walked back towards the entrance, from where the others were watching her and Spike; as she half-expected, her horn brightened again. She resumed walking towards the far end. Sunlight striking the exterior wall refracted and reflected throughout all the walls, the escaping prismatic light competing with the interior lighting. As before, her horn got increasingly dimmer the closer to the back she got.

“The anomaly appears to weaken my unicorn magic,” she said, her voice reverberating in the large, empty ballroom. “The further I go in this direction, the worse it gets.” Spike dutifully wrote that down. Back at the hallway, Rainbow Dash and Applejack exchanged uneasy looks; Princess Cadance had no visible reaction. “Compensation is possible via increased magical exertion.”

Time for a different experiment. The alicorn tried lifting her saddlebags with her magic and found it surprisingly difficult, as if they had gained a lot of weight. “Telekinesis is similarly affected.”

They reached the back wall. “To maintain light output here, I need ten times the normal exertion level.” Her number one assistant continued writing down her observations. The two walked along the back wall, searching for the point at which magic seemed the weakest.

They found it in front of a modest door, probably leading to a storage area. It was locked, both magically and physically, but that was easy enough to take care of—not quite as easy as it would have been if her magic was at full strength—but soon enough the door opened, indeed revealing a storage area with empty, shelf-lined walls.

But what immediately caught the alicorn’s eye was a mirror reflecting her lit horn. The glare made it difficult to see anything else, so she dimmed her horn. The only other light was from the ballroom behind her. Her eyes quickly adjusted revealing a familiar object. Why had it been moved here? Once she had passed through it to a strange realm, one populated by bipedal “humans.” It was at least another year before it opened again, so for now it ought to be “just” a mirror.

Twilight slowly crossed the dozen or so feet to the mirror, with each step her horn requiring additional exertion to maintain its brightness. The sound of her clopping hooves revealed the absence of reverberation, lending the room a claustrophobic vibe. By the time she had reached the mirror, it was taking unsustainable effort to maintain even a dim glow. “It sure looks like the mirror is the problem,” she told Spike.

The lavender alicorn in the reflection stared back at her. Might as well see what happens. She raised a hoof to touch the mirror’s surface. As expected, it met a solid surface—my horn went dark! In shock she jumped back; to her relief her horn resumed its dim glow.

“You okay?” Spike nervously asked.

“I-I’m fine,” Twilight assured him, her pounding heart calming down. “Just surprised, that’s all.” She touched the surface again, and again dark her horn went. This time she maintained contact and tried levitating her saddlebags. Nothing. She tried levitating her tail. Nothing.

Spike pointed apprehensively at her cutie mark. “Twilight…”

She saw the baby dragon’s reflection in the mirror and rotated so that she could see her reflected cutie mark. It was slowly fading. “That can’t be good,” she said, as she quickly broke contact with the mirror. Her horn resumed its dim glow and her cutie mark stopped fading; it actually began to darken, though more slowly than it faded. That makes perfect sense. A cutie mark is a magical projection onto the coat. Without magic to sustain it, it would have to fade. There’s barely enough magic here to restore the projection.

Twilight unlit her horn and again tried levitating her tail. This time it lifted, but her tail felt impossibly heavy to her telekinesis. She relayed all these observations to Spike, who wrote them all down.

“Let’s tell the others.” They started walking back. Her cutie mark continued to darken, returning to normal shortly after re-entering the ballroom. Twilight pondered what this all meant, with little success. The portal was closed, but it appeared to be suppressing her unicorn magic; this hadn’t happened even when it was open. In order to figure out what was going on, she needed a hypothesis to test. By the time she and Spike reached the entrance, an experiment had been devised.

The two found the other ponies just inside the hallway, waiting for their findings. Twilight got down to business. “The storage room back there has The Mirror,” she declared. “The one that Sunset Shimmer used to leave Equestria. It’s the source of the anomaly, isn’t it?” Twilight waited for Cadance to answer.

“Yes,” she confirmed. “I had it moved there to keep it out of the way.”

“What exactly is the anomaly doing?” asked Applejack.

“It’s suppressing magic. When I touched it, my magic completely vanished.”

“But only while you touched it, right?” Rainbow Dash anxiously asked.

“Only while I touched it,” she assured her. “I’m perfectly fine now.” Was that a foolish thing to do? What if it hadn’t been temporary?

Applejack didn’t look assured. “The mirror never did this before?”

“No,” Cadance replied. “Nopony knows where the mirror came from or how old it is, but nothing like this has ever happened during Celestia’s reign.” Her face became deadly serious. “Twilight, the affected area is growing. Slowly for now…”

Though she knew there could only be one answer, Twilight nonetheless had to ask the question. “My brother could not contain it?” Obviously not, or he’d be here containing it and not on a train to Canterlot. And he didn’t volunteer any information about this anomaly either, she now realized.

“He tried. His shield worked, but only temporarily. The smaller the volume contained, the faster it collapsed.” Of course. Shining Armor used unicorn magic. The shield was being eroded from the inside.

“There’s an experiment I need to perform,” Twilight stated. “Let’s go to the balcony.”

The ballroom opened up to a broad balcony. Twilight led the group outside into the late morning sun. The sounds of crystal ponies going about their lives drifted upwards from far below. Getting as far away from the anomaly as possible, she walked all the way to the railing. As the others joined her, she told them, “Leave this area in front of me clear.”

Twilight conjured a small cloud a foot off the ground, big enough to hold a single pony, from the humidity of the surrounding air. She was far enough away that her magic was unimpeded, as she had hoped. The cloud was positioned so that it had a straight line to the mirror. Faking a grin, she mockingly looked around and said, “I need the services of a pegasus. Any volunteers?”

The only pegasus present responded with forced enthusiasm. “‘Danger’ is my middle name!” With somewhat less enthusiasm, she asked, “What’cha got in mind?”

Twilight went serious; science wasn’t to be taken lightly. “Hop onto this cloud.” Rainbow Dash complied. “I want you to remain standing on this cloud while you use your wings to slowly push yourself all the way to the mirror.”

“What d’ya expect to happen?” she asked with a hint of trepidation. Known dangers are easier to pony up for than unknown dangers, figured Twilight.

“I can’t tell you; it could negatively impact the quality of the generated data.”

Rainbow Dash digested that for a moment and got indigestion. “Eggheads…” she quietly muttered under her breath.

Twilight pulled a device out of her saddlebag and gave it to Spike. It was rod-shaped, with a flattened rectangular head that possessed several knobs and a meter. “Take and record readings as we go,” she instructed. I won’t be able to use it once we get too close.

Spike grabbed it, turned it on and took his first reading. “99.9%, basically normal.” He wrote it down.

“Ah reckon that’s one of your scientific devices?”

“Sure is, Applejack. It’s a probe that measures the strength of the ambient magical field.” Her attention switched to the perched pegasus. “Okay, Rainbow Dash, let’s go.”

Rainbow Dash began to gently push herself forward with her wings, heading towards the mirror. Twilight exchanged a glance with Princess Cadance. Yeah, she probably shares my suspicions on what is about to happen.

Twilight, accompanied by Spike, started to follow Rainbow Dash and her cloud, remaining a few feet behind. Everypony else likewise followed Twilight. Spike took readings every so often and noted that the field strength was decreasing, slowly at first, but at an accelerating rate. It appeared to be proportional to the inverse square of the remaining distance to the mirror.

For over a hundred feet nothing in particular happened, but at some point Rainbow Dash started shifting her weight from hoof to hoof. “This cloud is starting to feel a bit funny.”

“Funny how?” asked Twilight.

“Well, it’s getting sorta, I don’t know, soggy.” Twilight was giving her one of those looks. “Yeah, I know it’s made of water, but—I dunno know… how to describe it… it’s like it’s starting to fall apart and, well, I don’t want it to fall apart, but it’s ignoring me.” Her weight shifting grew more aggressive as her footing became increasingly precarious. “And it’s getting harder to push with my wings.” Spike wrote it all down along with the probe’s reading of 3%. They were about 15 feet from the mirror. Twilight was all but certain as to what was about to happen.

The cloud was now visibly losing its cohesion as they entered the storage area, and Rainbow Dash was struggling to stay on top. She was also pumping her wings way too hard given her forward progress. 9 feet, 1%.

A few seconds later, the pegasus fell through the cloud, dissipating what was left of it. She instinctively tried to hover but to no avail; she fell to the floor. Fortunately it was only a few feet and she was shaken but unharmed. 6 feet, 0.5%.

“I can’t fly!”

“Nor stand on clouds or even keep a small cloud intact,” Twilight observed. She shook her head. “This is bad, very very bad.”

Rainbow Dash kept trying to take off but could do no more than hop a few inches off the ground, her flapping wings nearly useless. Twilight knew neither she nor Cadance would do any better.

Applejack couldn’t believe her eyes. “What about us earth ponies?”

Twilight was sure of the answer, but what experiment could she perform? There was no apple tree for Applejack to buck, and she certainly couldn’t conjure up one here—but then she had it. “Spike, put your quill down in front of Applejack.”

He did so.

“Now, Applejack, pick it up with your hoof.”

She tried, but she couldn’t lift it off the floor. “It’s just like when Tirek took our magic.”

Rainbow Dash walked over and tried to pick it up, also without success. “Yeah… remember how hard it was to turn those keys with our hooves?”

Spike tried his fire breath, but little more than sparks came out.

“I reckon none of us are immune,” Applejack concluded.

“Spike, go to the mirror and take a reading,” Twilight instructed. “Then touch it with the probe and take another reading.”

“On it.” Possessing actual claws, the absence of magic could not prevent him from picking up a quill or using the magic probe. “At one foot the field barely registers, under point zero one per cent.” He touched the probe to the mirror. “Zero.” He turned a knob, increasing sensitivity to maximum, and touched the probe to the mirror again. “Still zero.”

Twilight was satisfied; the experiment was over. “Let’s walk back. We’ll all return to normal once we get far enough away.”

Every few feet Rainbow Dash tried to take off, each time her wings providing more thrust; eventually she was able, with difficulty, to stay airborne. About fifty feet from the mirror, Twilight stopped and retrieved a different instrument from her saddlebag, this one a flattened box shape containing two meters, a compass-like pointer, and the usual collection of knobs.

“What’s that one do?” asked Applejack.

Twilight turned it on and set it on the floor. “The magical field behaves like a viscous gas. This device measures the flow and viscosity of the field. Levitating it messes up the readings, which is why I put it on the floor.” She waited a bit longer for the readings to settle down. “There’s no question about it,” she began. “Magic is flowing towards the mirror and the field viscosity is below normal.” Spike recorded the data off the device.

“I should take another reading closer to the mirror.” Twilight picked up the device, trotted a dozen feet closer to the mirror, with Spike tagging along, then set it down again. A half-minute later the readings had settled, showing that the flow was faster and the viscosity lower. The two rejoined the others, then all of them continued on to the balcony. Outside, the magic flow was minimal and the viscosity was normal.

Twilight walked over to the railing and leaned against it, staring off into the distance, letting it all sink in. Somehow the magic of their world was bleeding out; worse, the viscosity changes showed that something else was happening too—but what? How will she solve this one? Am I being tested yet again?

Applejack came up alongside her.

Twilight said, quietly, “You’d think I’d be used to it by now, but it doesn’t get any easier.” She continued to stare at infinity.

“We’re all here to help ya, doncha forget that.” She too stared into the distance.

Rainbow Dash drifted down in front of them, on the far side of the railing, forelegs crossed in defiance. “Why do I need magic to fly?!” she demanded. “Birds do it all the time and they aren’t magical.” She quickly added, “Most of them.”

Twilight looked at the pegasus in disbelief. “Wasn’t this covered in flight school?”

“Uh…” she sheepishly said, rubbing a hoof through her mane.

Twilight rolled her eyes. Probably too busy daydreaming about the Wonderbolts to pay attention. “Pegasus wings are too small,” she explained. “Wings of any size cannot provide both the raw speed and agility all pegasi have, never mind a pegasus like you.”

Rainbow Dash considered that for a moment. “I guess that makes sense. I always did wonder why birds had to work their wings so hard.”

Cadance joined them. “What now? Do you want to try your Rainbow Powers?”

Twilight looked down at the streets far below. She watched the crystal ponies going about their business, oblivious to the danger. Not that any purpose would be served by informing them at this time, other than to cause panic. Hopefully they will never need to know.

“Not yet,” she said with a sigh. “What worries me is that our magic will just follow the ambient magic to who knows where. I don’t think magic can reach the physical mirror itself.” She turned away from the railing to look at her fellow ponies. “If the ‘hole’ through which magic is draining is growing larger on its own, then using the Elements on it, forcing more magic through it, might only make it grow faster.”

“We do have some time,” Cadance said. “Assuming it continues to worsen at the current rate, it will be months before it seriously threatens the Crystal Heart.”

“Why not move it out to the middle of nowhere where it can’t bother anypony?” asked Applejack.

Cadance slowly shook her head. “I’m afraid it may be too late for that. Any ponies who attempt to move it will have to do so without using magic of any kind. It’s a long way down to the ground floor and the Crystal Heart is right outside.”

“Move the Crystal Heart?” Spike suggested.

“The affect that would have on the crystal ponies would be nearly as bad.”

“It wouldn’t be a permanent solution anyway,” added Twilight. “Given enough time it would bleed all the magic out of Equestria and beyond.” She didn’t add that the chances were good that at some point something would give, like a dam collapsing, increasing the flow exponentially.

“So let’s take care of it now!” insisted Rainbow Dash. “That way, we don’t have to worry about moving it.”

Twilight looked at the pegasus. You make it sound so easy. “Why don’t you go fetch the others.” It was time to get them involved. There had been no need for them to hang around as Twilight conducted her initial investigation, so Rarity was out shopping for fabrics, Pinkie Pie was shopping for a party, and Fluttershy was helping Pinkie Pie.

“On it!” Rainbow Dash vanished into the distance, leaving behind her rainbow contrail.