She stops before the open doorway. She’s already been surprised by how the cave seemed to have some sort of ambient light to it; she hadn’t needed her hornlight at all, all the way down.
Now, she’s standing in front of a wide-open doorway in the solid metal hide- and a simplified version of her earlier spell confirmed it’s the same metal hide as is in the bottom of that pit out there.
And her magic cannot get in through the open doorway, through which light is spilling. Bright, but gentle, white light- not unlike her hornlight, though she doesn’t feel any magic involved.
Which makes sense, if her magic can’t get through this odd barrier. This odd, invisible barrier.
She’d assumed before that it was something in the metal repelling her magic. Now, though…
She projects an even further simplified version of that earlier spell- one that’ll cost virtually nothing- against it, and lifts her hoof to touch it, very carefully.
The moment the tip of her hoof passes the edge of where her magic can reach, she pulls it back, and examines it magically.
Absolutely nothing changed.
So she tries again. Sticks an entire hoof past that point, draws it back, and examines.
Again, absolutely nothing.
Even though, even with the excellent thaumic conductor that is pony flesh, her magic can’t get any closer to the opening.
Her next test is an entire foreleg- and again, nothing.
So she tries the very tip of her horn. The unicorn horn is both a delicate thaumic instrument- and, simultaneously, the best known thaumic conductor. If it can’t get magic through the barrier, nothing can.
She lets out a small gasp as the tip of her horn makes contact with the barrier; she wasn’t prepared for the complete and utter destruction of the spell matrix that accompanied that act. She reforms it, though, still touching the barrier, and resumes the spell. The matrix had only collapsed at the point of contact; the rest had been a thaumic chain reaction, but nothing a prepared unicorn can’t handle.
She tries that for a second. She can still feel her power in the tip of her horn, even funnel more there, but it won’t leave for a spell, positioned right in the middle of the barrier- assuming it has a nonzero depth, of course. She hopes it’s not a field; that would be a… pain to deal with. Especially if there’s any negative side effects to prolonged exposure.
She draws the tip of her horn out again, tests it, and scans it. Nothing seems to have changed; immediately upon removal from the barrier, it resumed normal function.
So she tries again, this time progressing slowly through the barrier. She’s ready when she makes first contact, this time; as a result, her spell matrix remains firm.
Funny… the barrier seems to be about a third as thick as her horn is long. She successfully reactivates her spell, meeting no resistance for the five meter or so range, with the tip of her horn.
Then she cancels the spell altogether and steps through the barrier herself, into the doorway.
The very deep doorway.
Then she lights up her horn again, checking herself for damage. She’s not a doctor- but she spots absolutely nothing different than the scan she’d made before stepping through. Her disguise- since it’s dye-based, not magic- is completely unchanged.
Save only the tip of her tail; that’s still inside the barrier.
So she continues through the doorway, and out the other side, into a large, metal room, decorated with a few interesting-looking seats at the back- judging by the direction the seats are facing- and a comfortably soft rug.
“All safe, I assume?” an unseen stallion asks suddenly.
She lets out a gasp, shifting to a ready stance, and looks quickly around for him- even back the way she’d come. Nothing. Even her horn can’t find him; she’s the only pony, on this side of the barrier, for at least a hundred meters.
“I’m afraid you won’t find me in there,” he continues, “as a pony, at least.”
“Explain?” she demands, still braced.
“You see, I’m not a pony, but a machine. A very large machine, of which the room you’re in is a part.”
She relaxes her stance a little. “So, you can talk?”
“Yes.”
“And you didn’t talk to anypony outside?”
“I judged that would unduly frighten them, especially seeing as how said outside came to be revealed in the first place.”
She blinks, fully relaxing her stance as she tries to trace the voice; it seems to be coming from everywhere. “Speaking of that revelation, I hear it happened in a large explosion?”
“Yes.”
“And that that explosion came out of that massive stick-thing overhanging half the digsite?”
“That’s an affirmative.”
“... And that stick thing is part of you?”
“You’re probably the first to make that connection.”
She lets out a brief snort of laughter. “I take it Twilight doesn’t know about your cave?”
“Oh, she’s been in here. I believe she’s convinced the two locations are separate machines.”
She chuckles. “Had I not traced that cave by magic, I might have thought the same,” she states. “Deceptively long. Um… May I ask why such… uh, violence, was used in that pit?”
“At the time that I fired that main battery Hellbore, the trench that the tip of the weapon was exposed in was filled to bursting with what I believe were called ‘Timberwolves’. Additionally, I had four ponies held safely behind the firing chamber- one adult, and three youth. At the time, I had not made any contact with ponykind, and believed that allowing them to die while I could protect them would have been a bad start.” He sighs. “Completely aside from how I detest unnecessary civilian deaths- even among my enemies.”
“... Ahh. That… kinda makes sense, actually. And while ponies were frightened like crazy, the way I hear it, nopony was hurt. So…” She glances up. “If the town’s seen worse than what I did, that’s nothing.”
“What you did?”
She blinks, and stuffs a hoof in her mouth. “Oops.”
He chuckles. “Oh, don’t worry, I won’t be telling anypony. You’ll want to be careful where you slip, though- I’ve identified at least one secret investigator in town.”
“Ahh…,” she nods slowly. “Okay?”
“Speaking of which, I have reason to believe she was harmed by my first attempt at a thaumic barrier. I’ve since spotted her out and about in town, though her demeanor seems to have changed, so I’m interested in a second opinion- that is, if you can get it without provoking suspicion on her part- or on the part of her unicorn partner.”
She winces. “That… could be difficult. Um…” She glances back at the door. “Did you say the first one hurt somepony?”
“I have reason to believe it did, yes. However, that one was formed by tuning an antikinetic barrier against thaumic energy; this one is a dedicated system, designed for the purpose. I’ve tested it a few times, and it appears safe to me; did you notice anything in your examination?”
“Notice anything? You mean, danger anything?”
“Yes.”
She nods. “Ahh. No, it’s completely safe, as near as I can tell. I can’t be certain about long-duration exposure, but at least brief exposure is harmless. It does kinda shatter any spellwork, though.”
“So, works as designed. Thank you.”
She chuckles. “What? Afraid they’ll tear you apart?”
“If given the opportunity, I have no doubt Twilight would pull me apart, piece by piece, to see how I tick… even though I have a complete copy of my own blueprints, and can print them off for her without issue. However, I was not aware of Twilight’s propensity to do such when I designed the barrier; at the time, friendly status of ponies was not confirmed, and I needed a method by which to protect myself against any hostile action. Even now, the main reason I keep the barrier online at all times is because I have reason to believe the weapon that originally exposed the tip of my main battery- and damaged it at the same time- was magic-based.”
“Reason to believe?”
“Yes. I am told it was Tirek, using some sort of magic laser beam; however, a Unit of the Line such as myself does not rely solely on word of mouth, one of the most inefficient and inaccurate methods of information retention and transmission. Besides, where one high-powered thaumic enemy arises, so can another.”
“... Ahh. That makes perfect sense. So… Let’s see. You’re hiding what you really are, yet trying to present yourselves to ponies as an ally, when you’re… well, a walking weapon, right?”
“I don’t know that I’d describe myself as a walking weapon, per se; rather, as a mobile fortress. Yes, I have weapons beyond any I’ve seen yet on this world; I am also heavily armored… and I ride not on legs, but on tracks.”
She lets out a chuckle. “Well, alright. But I got the rest right, right?”
“Yes.”
She puts a hoof to her chin. “How possible would it be for you to, um, create a pony-body for yourself?”
“An avatar?” he asks. “As a matter of fact, I already have two of them deployed- hence how I might be able to deflect suspicion in the requisition of that second opinion. The issue is, I’d like to establish friendly relations between my hull and ponykind as well, not just with pony look-alikes.”
“Ahh… How’s that coming?”
“You’re the third pony to come in here. And, to be honest, the first to come without assistance; I used invisible, psychoactive lamps to attract first Fluttershy, then Twilight. Both have rested in this room; the former appears to have a powerful propensity for kindness, while for the latter, I went with Tactical and played psychologist to help her decide how to handle her new home. That castle is new, right?”
She blinks. “Ahh… Makes sense. Fluttershy is the Element Bearer of Kindness, and Twilight has always been easy to distract. Um- you didn’t use those lamps on me, did you?”
“I did not; I had no reason to, as my sensors have no issue detecting thaumic activity. I knew you already knew where to find my cave; all I had to do was disable the holoprojector- and physical barrier- covering the entrance, and power the ambient lights inside the tunnel itself. That last of which I probably didn’t really need to do, since you’re a unicorn, but they help with the friendly feeling.”
She blinks. “Makes sense,” she states. “Um…” She glances at the door. “Do you know about the thaumic stability problem?”
“Thaumic stability problem? I suppose it’s a good thing I’m protecting myself against it, then.”
She chuckles lightly. “Ah, no, it’s not like that. It’s more about the stability of the common spell- the reason it takes so much effort to work a powerful spell.”
“Then no, I’m not familiar with it.”
“Well…” She sits on the floor. “It’s an interesting problem.”
“Those seats are freely available, if you would like,” he interrupts.
She glances at them. “Um… Okay, sure.” She rises to trot towards them. “Anyways… When a pony uses magic, some of their power leaks out of the matrix. The more power they’re using, the more leaks out. Um… Wow, these things are really comfortable, thank you! The problem is, the power that leaks out wreaks havoc. Both on the spell matrix- increasing the difficulty of the cast- and on the caster. Pegasi and earth ponies are generally immune to this effect, since their innate magics don’t actually take that much power at all; it’s unicorns- really only the powerful ones- that have to worry about the damage to their bodies.” She sighs, eying the tip of her horn. “I’m one of those ‘powerful ones’. If I use too much magic in too short a period, I’ll cause myself some rather painful injuries- and, eventually, kill myself. That’s…
“That’s how I lost my parents. Both of them.
“Magical artefacts, like the Elements of Harmony, aren’t immune to this effect either- and while the artefact itself isn’t bothered by the lose magic, the wearer slash user is.” She sighs. “Twilight Sparkle, Rarity, Rainbow Dash, Fluttershy, Applejack, Pinkie Pie. Those six ponies regularly use the Elements of Harmony… and overuse them, too- so they’re killing themselves, without even realizing it. That’s… That’s what I’ve been trying to solve. I’ve been trying to find a way to inform them about the danger, and to heal what damage has already been done.” She looks up at the ceiling. “I managed to get the healing spells on Fluttershy, but none of them will listen, so it’s not going to last very long.”
“That would explain why Twilight was carrying serious health issues around, while Fluttershy merely looked a little exaggerated.”
“Huh?”
She can swear she hears a smile in his voice as he continues. “Well, what if I told you that comfort is not the primary function of those seats?”
She blinks. “I’d tell you the primary function better be something incredible.”
He chuckles. “I daresay it is. The primary function is to keep passengers in place while I move.”
She blinks. “That’s incredible?”
“The secondary function is to serve as a delivery pathway for something called ‘nanites’, with which I can formulate a medical diagnosis- and operate repairs. Speaking of which, it looks like you’ve got some pegasus genes; you’ve got a partially-developed mid-shoulder. Doesn’t look like it plans on growing further, though.”
Silence holds for ten seconds, before she nods.
“Alright, I’ll give you that one. That is kinda incredible.” Then she glances at the door. “I don’t suppose both Twilight and Fluttershy have, uh… used the seats?”
“They have. I’ve restored both of them to perfect health- or close thereto. Twilight had to leave before I got that far.”
She chuckles. “How long does it take?”
“Depends on the problems I find. In case you’re wondering, I’ll have you done in about fifteen minutes.”
She smiles. “Thanks. So, if we get them all in here for… healings, then… Do you think you could help with the education side of things, as well?”
“Possibly. That’ll be down the road; I don’t think they trust me enough to accept that kind of advice just yet.”
“Though, if they learn to associate you with their good health…”
“No doubt they’ll ask me if I can make the good health last- which I can. That procedure is a little more invasive, though, and not something those seats can handle.”
“What’s it involve?”
“Surgical installation of a medical suite, with chemical power plant, within the pony body.”
She tilts her head. “What effect would it have?”
“You’d heal faster… and that’s about it. Material density is easy to match to the surrounding tissues; the power plant would only burn about as much in the way of food as the same biomass as was removed would. Plus, the footprint of the device is small enough a doctor might miss it- and that installation won’t risk damaging any organs or tissues, other than that which is replaced.”
“What would be replaced?”
“I’m not entirely sure- I don’t have enough data on pony biology just yet- but something inconsequential.”