Flameless torches burst to life, their light dwarfing the glow of Celestia's horn to reveal a large, dusty room, its only apparent feature a short, but wide, square pedestal rising from its center. Over a dozen unicorns drain down the stairs into the room's corner after her, the group maintaining Sombra's current prison between them while they form a circle in front of the room's entrance.
"Stay where you are," Celestia instructs, making her way to the pedestal. Walking in a slow circle around it, she gives each of its four corners a tap with her horn.
A rune glows to life upon each touch, the magic held within them burning through the room's thick layer of dust with an ominous hiss to leave the dull orange symbols clearly visible. As soon as the fourth rune is lit, more runes begin to glow as well, a column of symbols activating beneath each corner to reach the floor in perfect synchrony with another revolution of Celestia around the pedestal. Celestia continues her walking, more runes activating to form increasingly complex layers of enchantments as she spirals further and further outward, the hissing becoming louder and the smell of burnt dust more poignant with each step. Coming to a stop just in front of the circle of mages, Celestia spreads her wings and takes flight, hovering halfway between the pedestal and the room's corner.
Her horn glows brighter, the aura of her magic surrounding the shield her sister had used to contain the former tyrant, "You may release him now."
A unanimous sigh of relief echoes from the unicorns as their horns wink out, several slumping to the cool stone floor, their exhausted groans interrupted by the occasional sneeze. Celestia meanwhile delicately levitates Sombra over to the pedestal, sweat beading on her face and sparks of magic popping from her horn by the time he touches down, the surface of the pedestal deforming until it matches the curve of the barrier holding him perfectly.
As soon as Sombra is in place, the outermost runes shine increasingly brightly, the room's torches flickering and going out as magical power rapidly spirals inward along the runes towards the pedestal. A brief pause follows once all the runes are charged, the room lit almost entirely by their dangerous glow save for the beacon of Celestia's luminous horn, the air thrumming with structured magical energy. Suddenly, an orange fog engulfs Sombra's prison, the haze solidifying into a shield an instant later to shatter Luna's and take its place.
Celestia cries out at this and her wings falter, magical feedback searing through her horn as she tumbles towards the rune-laden floor. A hodgepodge of five auras surround her just before she falls onto her own massive enchantment however, an equal number of unicorns gritting their teeth while they focus what remains of their magic to lift their princess to safety.
Celestia's legs tremble once her mages lower her to the floor between them, though she manages to remain standing. Four of the unicorns who saved her are less fortuitous, simultaneously collapsing into sweaty heaps. The fifth unicorn gasps, sitting heavily, but with a concerted effort is able to keep his muzzle from hitting the same dusty stone as his colleagues.
"Princess… Celestia," he gasps out between breaths, "if you… don't mind me asking… what is this place? Who made it?"
"I did… some nine and a half centuries ago," Celestia replies, sweat dripping down her body. "You are Solar Flare, correct?"
"I – y-yes," the stallion stammers. "Um… may I ask why?"
Celestia rubs her eyes and sighs, "After my sister… succumbed to dark forces, I feared that one day I would also fall to the same temptations. And if I fell… there were no other alicorns to deal with me, no others able to wield the Elements of Harmony. So I made this place… by tapping into the ley line running beneath Canterhorn Mountain… which is also why I originally decided to have Canterlot built where it is." She laughs tiredly, "Though you will not find that explanation in any history book."
Forcing her hooves into motion, Celestia turns to face Sombra, wisps of magic flickering along her horn to gather at its tip. After half a minute, Celestia lowers her horn to the closest rune, the fiery symbol absorbing her magic instantly. A split second later the entire runic spiral pulses, sending a shockwave of magic deep into the earth. For another half minute, nothing happens.
Then the runes glow red.
Without any further warning, a massive upswell of magic slams into the runes from below, the spiraling engravings blocking and channeling the power into Sombra's new prison, turning it into a blazing inferno of raw elemental energy. The surge peters out only a few seconds later, the runes returning to their normal orange glow as if nothing had happened.
Solar Flare sits gaping at the display, afterimages of the runes still visible on the insides of his eyelids with each blink, "Sweet – uh – you. What was that?!"
The other unicorns, tired as they are, nevertheless look to Celestia in search of an explanation as well, each of them equally awestruck.
"That," Celestia answers, "was a fail-safe, in case even these wards were unable to contain me on their own."
Princess Celestia watches carefully while the brilliantly shining orb at the room's center dims, the elemental maelstrom fading until the barrier appears empty of both magic and evil smoke alike. The resulting void is short lived however, her eyes narrowing as Sombra's telltale smoke reinstantiates.
"Uselessss… " Sombra goads her, laughing.
"Insolent prick," Celestia mutters to herself.
"But… but h-how is he… ?" Solar Flare stammers, flinching as Sombra's gaze turns to him.
"Both the natural and magical worlds are governed by unbreakable laws, many of which I would hope you are quite familiar with," Celestia says, stifling a yawn. "The more mundane rules such as the various and sundry thauma-to-matter conversion ratios, for example, all the way up to those governing life itself. For him to survive such an assault, it must be that Sombra has found a way to twist himself around one of these laws in order to anchor himself in this world, regardless of how much magic we throw at him. But-"
Celestia yawns again, turning to send a baleful look up the long flight of stairs, the silken promise of her waiting bedsheets like a siren's call to her body and mind alike, "But unbinding him is now more a matter of tedium than difficulty, and as such is a problem for another day. He is not going anywhere for a few centuries at the very least."
"If that is your wish, your highness," Solar Flare replies, sending another nervous glance Sombra's way before trailing Celestia up the stairs, the other unicorns dragging themselves to their hooves to follow in their wake.