Almost as soon as Twilight and Starlight stepped through the doorway, it once more swirled in on itself. The other doors did much the same, and as the fillies watched, they once again became beams of light that shot back into the shining orb above them all.
The orb flashed brightly a few times before it winked out entirely, bathing everything in an inky blackness for a few brief moments. The darkness gradually receded around them and soon enough, the light of a magic chandelier began replacing it.
Within a matter of seconds, Twilight, Starlight, and Aeon found themselves in an empty room consisting of nothing but a stone floor and stone walls, and a wooden door.
Before the fillies could wonder too much at what had happened, Aeon quickly made her way to the exit.
"Alright, come on you two," Aeon urged, as she pulled the door open with her magic, "while time may dilate in the Nexus, it's not dilated by much, so we've still been gone for a lot longer than I would've liked."
Normally, a pony like Twilight would've jumped on the subject of time dilation and its effects, but as it stood, she and Starlight were far too distracted by everything, and everyone they had seen.
With the fillies lost in their thoughts, neither of them said a word to Aeon as they trotted back down into the library. Aeon, was content to leave the girls to their musings as she led them past row after row of bookshelves.
The girls slowly began to return to the real world when each of them realized they weren't being led back to their bedroom for the night. Instead, Aeon had led them out of the library and down the hallway to another part of the Fortress.
"Um... Aeon?" Starlight asked, glancing at the grim looking residents passing them by with a nervous frown, "where exactly are we going?"
"I got one more thing I want to show you girls before you turn in for the night," Aeon replied, her tone and expression serious, "it isn't gonna be pretty, and it probably won't help you get any sleep, but I think its high time you saw what the desert has to offer."
Aeon said nothing more beyond that ominous statement, and Twilight looked over to Starlight worriedly. They traveled for a few more minutes and Twilight began to get the sense that they were heading deeper into the castle.
Neither Twilight nor Starlight recognized the area they were being taken to from their tour with Burning Comet, and Aeon gave no explanation as to where they were headed.
They eventually turned a corner and came to what Twilight at first glance, thought was a dead end. The hall ended in a wide, dimly lit circular area, and upon closer inspection, the fillies could see three different doors.
Standing in the middle of the area, there was one door to their left, one to the right, and one massive set of double doors directly in front of the three ponies.
Each door to their left and right was made of a plain dark metal and aside from that, was otherwise unremarkable. The middle doors however, were anything but.
The double doors together were made of a slightly brownish grey metal and easily five ponies tall and at least six ponies wide. It was a beast of a door, with layers of metal around the edges, bolts drilled into the rim of the doors, and a large, strange mechanism covering the center.
The strange device looked like an oversize black cog engrave with yellow, softly glowing arcane looking symbols that completely baffled the lavender filly.
She tilted her head in thought for a moment before her eyes widened in recognition. She turned to Starlight to see the light pink filly distracted by something else.
Two guards stood vigilantly on either side of the door. They were clearly ponies, but the hooded black cloaks they wore obscured their tribe from view.
Starlight could've sworn she saw fangs peeking out from their dark grey muzzles as she, Twilight, and Aeon got closer. Before she could glean anymore however, she was pulled away from her thoughts by a poke from Twilight.
"What?" Starlight hissed, rather annoyed by the distraction, "what do you want, Twilight?"
"Those doors with the giant cog thing in the middle," Twilight replied, ignoring Starlight's irritated tone as she pointed at the doors in question, "there's some weird writing on the cog. Is that Runic Magic?"
Starlight raised an eyebrow and turned to the door, blinking in surprise at what she saw. Aeon glanced down at the two fillies and grinned slightly, but didn't say anything as Starlight's eyes scanned the doors just ahead.
"That's... definitely Runic Magic at work alright... but," Starlight frowned in bemusement, and narrowed her eyes, "I'm assuming the runes are forming some kind of Binding sigil, but I don't recognize any of the runes being used."
"Well, you're right at any rate, Starlight Glimmer," Aeon said suddenly, "that is most definitely a Binding sigil, and very powerful one at that."
As they approached the door, Aeon stopped and, like Burning Comet had done with the minotaur guards before, Aeon nodded towards the cloaked ponies.
Without a word, the two ponies stepped away and towards the walls on either side of the large double doors. They each raised a hoof in sync and pressed them against the wall, causing two previously unseen miniature glowing yellow magic circles to appear on both sides beneath their hooves.
Twilight watched in awe as the magic circles started to spin, gradually picking up speed for a few seconds before stopping abruptly. The circles flashed twice, and Twilight turned to the large doors just in time to see the large sigil on the mechanism glow brighter and flash twice in response.
"Might wanna take a step back, fillies," Aeon said as she herself did just that, "this part can get pretty—"
There were two loud, muffled clacks that seemingly came from the doors themselves. The two fillies scrambled back in surprise as the cog began to spin slowly.
The spinning was accompanied by a whir that increased in volume as the cog began to spin faster. This soon turned into a piercing metal screech that, while only lasting a second or two, had the fillies ears ringing painfully.
The cog soon slowed to a stop and seemed to press itself into the door. For a few heartbeats there was nothing but silence.
Just when the fillies thought it was over, the doors suddenly gave a loud, drawn out hiss and the screeching returned as they slowly slid apart. After what felt like an eternity, the doors finally stopped with another clack that made the fillies jump slightly.
The doors themselves had somehow been pulled into the wall, leaving the path before them all wide open. The two cloaked guards gave Aeon a sharp salute before returning to their post wordlessly, standing on either side of the now open doorway.
"...So as I was saying, that part could get pretty loud," Aeon finished with a smirk at the two stunned fillies, "just a heads up."
With that said, Aeon casually trotted into the room. Twilight and Starlight remained where they were a moment longer, Twilight's shock turning to apprehension as they saw what was inside.
Starlight on the other hoof, was completely floored at what she was seeing.
The room itself was massive, and one thing that stood out to the girls as they reluctantly followed Aeon in was that both the walls and the floor within were made of some kind of smooth jet black stone.
The stone seemed to reflect the bright golden glow of the many, many runes both slowly circling a gigantic black orb floating in the center of the otherwise empty room, and the equally large and fiendishly complex looking magic sigil on the floor directly below the black orb.
The moment the fillies stepped into the room, something began to feel off. It took a second to figure out what it was, but a moment later, Twilight realized it was mana.
Raw, ambient mana permeated this room like a thick fog, and it made the lavender filly's horn tingle uncomfortably.
Starlight felt much the same, and the two couldn't help but notice that Aeon didn't seem affected in the least. Returning her attention to the large circular sigil on the ground, Starlight spoke up, addressing the cobalt unicorn mare.
"Aeon... what is this place?" the pink filly's gaze drifted to the orb floating above the sigil, "and what is that?"
Twilight didn't speak as she waited for Aeon to respond. Most of her attention had been on the orb since they entered the room. An unpleasant sensation twisted in her gut as she stared at the object.
She narrowed her eyes as her mind worked to figure out just what it was, and a moment later her eyes widened once more.
"It's a barrier spell," she nearly whispered in awe, before turning to Starlight, "it's a barrier... but it's not like any kind of barrier I've ever read about."
"And the purple one gets a gold star," Aeon exclaimed, smirking as she stepped up next to the two fillies, "this is the Black Barrier. I know the name is a little on the nose, but it'll most likely be the most powerful piece of Barrier Magic you'll ever see—aside from the Celestial Barrier that is."
"And... you're using it as a prison... aren't you?" Starlight guessed with a worried frown, her eyes glued to the orb, "there's something in there, isn't there?"
Aeon's smirk fell away and a solemn frown crossed her face as she replied.
"I based the Black Barrier off of the spell Celestia used to create the Celestial Barrier—albeit with a few of my own personal touches," she explained, trotting around the orb, "unlike the Light Magic that makes up a large part of her barrier though, I used a combination of Dark and Runic Magic to make this work."
Aeon made a full round before coming to a stop in front of the two fillies. She stood facing the barrier that took up most of the room. She stood there silently for a moment before turning her head to look back at the girls.
"I don't have as much raw power as an alicorn, so I can't cover nearly as much distance as the Celestial Barrier," Aeon continued, her expression turning grim, "but I can at least make an impenetrable prison for the things we can't kill, can't send back to Tartarus, or simply want to study to find a weakness."
"So... there's a... demon in there?" Twilight asked, taking a step back from the barrier in alarm, "but... you said this barrier used, Dark Magic. From what I've read, demons aren't affected by Dark Magic!"
"Attacking a demon with an offensive Dark Magic spell may not yield much," Aeon replied with a nod, "but demons, despite Dark Magic making up a majority of their life force, can still be affected by Dark Magic in other ways."
"Like how?" Starlight asked, eyeing Aeon intently, "how would you stop a demon with Dark Magic?"
"By making it work against them," Aeon answered, returning her attention back to the barrier, "the Black Barrier is special in that it adapts to a demon's unique magic signature and strengthens itself accordingly."
She gestured to the large magic circle below the floating barrier as she continued her explanation.
"That's the affect of the sigil you see below," she then pointed towards the barrier itself and the many runes slowly rotating about it, "my Dark Magic spell draws out the innate Dark Magic of the demon and the runes create—or rather imitate the effect of—a barrier spell and reverse its effects."
"I get it," Twilight replied, nodding slowly as she eyed the barrier with a thoughtful frown, "so you're basically forcing the demon to cast a Barrier spell using its own innate Dark Magic and flipping it around so that it keeps them in instead of keeping everything else out."
"Precisely," Aeon replied, "you catch on quick, Twilight Sparkle."
"Okay, that I understand," Starlight chimed in, tapping her chin and frowning in contemplation, "but that still doesn't explain how it's effective. The barrier might be reversed, but it's still Dark Magic and therefore should be ineffective."
"And that just goes to show how little you know about Barrier Magic, Starlight Glimmer," Aeon replied with a disappointed frown, "we'll need to fix that little problem as soon as possible if you want to continue living a long-ish life out here."
Starlight merely huffed in annoyance and looked away. Twilight turned to Starlight, her eyes practically sparkling as she straightened up and raised a hoof.
"You see, Starlight," the eager lavender filly began, "when you cast a barrier spell properly, it creates not one, not two, but three layers.
"An outermost layer that repels, a middle layer that specifies the type of magic or object being repelled, and an innermost layer that reinforces the barrier.
"While the innermost barrier isn't completely necessary, it does keep the barrier far more stable, and if your magic is strong enough, you can even have multiple reinforcement layers.
"In this case, from what I can understand, Aeon's runes somehow forced the demon's innate Dark Magic into a proper barrier spell, which means—"
"Which means all three layers are present," Starlight finished with a nod, "and since Aeon's spell drew out only the Dark Magic, that's what the second layer is targeting and that's what the outermost layer is repelling."
"And I'm willing to bet the sigil below is working in tandem with the runes above to affect the layer of reinforcement!" Twilight added excitedly, all worries about what the barrier actually held now completely gone. She turned back to an amused Aeon with a bright, hopeful smile, "am I right?!"
"Got it in one, filly," Aeon replied with a shrug, "see, this is what I'm talking about when I say you two are meant for each other."
Her amused grin suddenly vanished and her expression became unreadable. The change was so abrupt, that it made both fillies' hearts skip a beat. Aeon stared both of them down with that unnervingly blank expression for a few more moments before she blinked, and her eyes shone with a brilliant golden yellow light.
"Now who wants to see what's inside?"