The split was almost satisfying to watch, complimented by a low, guttural rumble that shook the entire cavern's structure, small discordant lines beginning to form along the base of the surface, which then bathed the previously unlit cave in a garnet red light. Stalactites were exposed to the eye for the first in many lifetimes, their bodies laced with the deepest obsidian and indigo hues.
The streaks running along the lines of the jagged walls contrasted beautifully with the still growing cracks, which shone brilliantly, and with further intensity as they became more prominent in the stone. The iridescent light, the many visual wonders, they would serve to mesmerise and stupefy any that bore witness to them, and mesmerise they did.
Eventually, the cave's walls were tremulous, although it could not be said if the occurrence was steady or not. There was no concept of time in this place. The darkness was so strong, so heavily juxtaposed by the heat and radiance of the colour below, that if an onlooker were to close their eyes, they would likely see the scintillating kaleidoscope of light emanating from the surface below. How far below, it was impossible to say.
With the unfathomable passage of time came an increase in the magnitude of shaking, the crust slowly threatening to give way under the rhythmic pummeling of whatever forces lay below, wherever the quakes were birthed. The rocks hanging above teetered on the edge of oblivion, stolid fear plastered across their non existent faces. Should they have known anything, they would know that they would plummet soon.
The beat that the walls of the cave now unwittingly danced to was zealous, damaging, and inevitable. With each fresh thud, the glow of the cave's floor brightened, burning embers lifting, floating on an impossible breeze. There was no air in this place.
Of course, this did not impede the fire, which began to spout from the growing fissures in the cave's floor, seeming to expand with the walls of the unnatural grotto, filled with nature as it was. The result may have perturbed the remnants of the old setting, but they had no reason to fear, they would be a part of the lava soon, which the fire promply gave way to.
The fire was relentless, and moved in tandem with the earth's bouncing trance. The lava seemed to be rising, crawling up the newly formed crevasse and spilling out onto the new floors, cauterising the open, previously unharmed holes that might have been found in the serrated stalagmites that populated the varying levels of floor. Still, the centre of the surface held its ground, even as the larger cracks began to intersect, and the ground threatened to erupt.
When the earth blinked, the shuddering stopped. The lava began to recede, the cracks slowly closed in on one another, meeting their counterparts once again, and the effects of the core's convulsions began to fade.
It was almost satisfying to watch.
Then, an explosion of red light, and the lava splashed the surface with violent force, thickening and coagulating as it began to rise with vigour unlike what it had before.
With a blanket of lava coating the surface of the cave, the most resilient stone was pushed out of the way like many dead leaves. What rose from the abyss was unlike any manifestation or incarnation before it, nor after it. It was incomparable, despite its appearance being marred by the sheet of lava coating it as it slowly rose.
One thing, and one thing only could be described of the creature that continued to rise: Terror, sheer terror.
The fractures in the crust of the deepest underground cave matched the newly formed fractures in Princess Luna's mirror.
***
Luna panted, shuddered, and shook, hoof inexplicably outstretched. It took her almost ten seconds to register her own consciousness, and another twenty to notice her current pose. Opening her eyes, she allowed them to trail down her foreleg to her hoof, which was still pressed against the cold mirror.
Removing her hoof, she realised that it was shattered, which only served to remind her of the horror she had just witnessed. After a few seconds of consideration, she realised how messy things could have been if her head had still been inside the mirror when she had smashed it. Decapitation was difficult to come back from.
Luna turned, unable to bear the taunting shards any longer, and made to walk across her chambers. Another few seconds, and Luna realised a guardspony had been standing at the entrance of the doorway --around ten paces from her-- for some indeterminate amount of time. Luna scowled.
Just what we need, attention drawn to us. We will dismiss this guard swiftly. Luna turned, straightening her back and assuming her most regal, intimidating pose, addressing the guard. "You. Why are you standing there?" she demanded.
If the guard was bothered by this, he did not appear so, stoic as he was. "There was a smash in your chambers, your highness. I came to investigate out of fear for your safety, and found you unresponsive when I entered, for at least thirty seconds."
Luna flared her wings, her expression incredulous. "You think we do not know this? You would do well to remember that we preside over the night, we are in full possession of our faculties, and quite capable of functioning in our own bedchambers."
The gaurdspony didn't appear satisfied with her response, as his present frown seemed to deepen, ever so slightly.
"If you must know," Luna continued, "We detected an impurity on our skin, and upon closer inspection found the mirror's verdict to be false. Of course, the mirror is defective, and is of no use to us."
This gave the guard pause. "So... You smashed it?"
Don't give him an inch. Luna nodded with conviction, daring the guard to challenge her. "Precisely."
Apparently, he was slow on the uptake. "Why didn't you just, you know, throw it away?"
"That is a good question," she stepped closer to the guard, speaking slowly and deliberately. "Your presence is also of no use to us, what should we do with you?" her voice was almost silky, annoyance protruding from each syllable uttered.
An audible gulp was all Luna needed to hear; the bobbing of the guardspony's rather large adam's apple was a bonus.
Spraying her wings, her voice became cold and commanding once again. "Return to your post. If your assistance is required, you will be notified. Thank you."
When the previously steely guard left, his movements seemed clunkier, and a tad frantic.
Luna would almost be amused by the spectacle if she wasn't busy releasing the breath she had been holding. Had that guard been more suspicious of her actions, he might have notified Celestia.
Letting her body slump, she lifted her front right forehoof, inspecting the shards of glass her silver horseshoe had failed to block, shards she had walked on without complaint in an attempt to deal with the guard without drawing attention to her injury.
Allowing a dark aura to envelop her hoof, Luna quickly removed the glass, repairing the sinew of the small wounds, knowing the external knitting would leave the hoof slightly weak for a few hours. If she had a minute or two spare, she would be able to completely repair her hoof without much effort, but now, time was of the essence.
The time was now. Luna had seen it in her scrying, she had identified that which she had predicted would come, which she had feared and convinced herself was mere legend. Since her return, Luna had silently been formulating a plan as of how she might deal with these horrors, as impalpable as they were. Her plan was shaky, unfinished, and above all, risky, but she had little choice now.
If Luna did not act immediately, there would be no other opportunity to do so. If the matter was not handled with the utmost care, all that she and her sister had achieved in their long lives would be for naught. No longer would she have to protect her subjects from the terrible, bone chilling assault of night terrors.
She would have to protect them from far more real, tangible nightmares. Nightmares that could touch, conquer, and kill with impunity, nightmares that would see no distinction between night and day.
There would only be fire, offset by the most captivating darkness and woe imaginable.
She would gather them first, then she would slay them, bind them, destroy them.
Stepping onto her balcony, overlooking Canterlot in its nighttime splendor, Luna ignited her horn, casting out her consciousness, searching for her first recruit, the first that could assist her in her time of need, that must assist her.
Otherwise, all would truly be lost.