Slowly they climbed the long stair.
It was a nightmare journey. Luna had silenced their hooves, but the thaumic radiation from a spell strong enough to let them gallop silently up those stairs would have instantly revealed them to the Dragon. So they stepped carefully, the glow from Luna's horn damped to little more than a glimmer, in which their images of the stairway were colorless and grainy, even given the night vision spell. They had to be careful of accidentally kicking some piece of debris, lest the clattering draw the Dragon's attention.
Luna was confident of her own ability to survive the consequences of its wrath, but feared that Summer Lightning might come to harm. She seriously considered sending her companion back -- she could teleport her into open air away from the mountain with reasonable safety -- but she wanted to keep her in reserve for as an escort for the Dragon's hostage. In her mind, Luna balanced benefits and risks of her various possible strategies, as she always had aforetimes, and as aforetimes knew that if she made a mistake, a friend might die for it. Such was her destiny.
Out of the underworld of forgotten antediluvian science they climbed, the Alicorn Princess and the Pegasus Ranger, on a stairway over four thousand years old, and they knew that a monster was at the other end, listening intently for them, ready to strike without mercy at their first mistake. It was a climb out of a horror tale, and it was real, and perhaps the strangest thing was that this was far from the first time that Luna had done something like this.
First time for Summer, I ween, Luna thought, and glanced at her companion.
Summer looked a bit frightened, but she was stepping firmly, with care and determination. She caught Luna's gaze and flashed her a wan grin.
Luna smiled back. I need not fear that the Ponies of my Sister's utopia have lost their spirit, judging by this one.
The stairs spiraled slowly around the launch tube in turn after turn. Luna knew the circumference and length of the tube, and approximately where it intersected the Dragon's caverns. The rest was eidetic memory and the super-equine capacity for visualization which came with being an adult Alicorn. Hence, she knew precisely how close they were approaching to the height at which the Dragon had breached the launch tube.
She might have known anyway. As they got about halfway up the tube, they began to encounter small pieces of debris which had rolled down from above. Drifts of dust, then pebbles, and finally boulders of native rock. They had to step carefully to avoid dislodging them.
They were dangerously close to the Dragon now.
Luna signaled a halt. She perked up her ears. There was complete silence on the ancient stair. Then there was a sound.
It was a great breathy whispering susurration, accompanied by a faint metallic scratching. Luna knew what it was. She looked at Summer, and saw her companion's eyes widen when Summer came to the same realization.
It was the Dragon, breathing.
Stay here, Luna motioned Summer, and very, very carefully, she climbed the last part of the stair.
It was partly blocked by debris. She could see where an ancient crack, the product of some unknown violence, had sheared through the armored double sleeve of the launch tube, unknown millennia ago. The crack started abut midway up the last part, and from it a rift widened into the living rock. She stopped, forcing herself to remain calm, something not entirely easy under the circumstances even for herself, and she smelled fresh air wafting in from the crevice, proof that it provided admittance into caverns which in turn connected with outside.
The air was, of course, not entirely fresh. There was also a strong archosaurian smell, like that of a great bird or crocodile. She was quite familiar with such a scent. It was the scent of the Dragon. But it was much weaker from the crevice than from the stairway up. The crevice clearly entered the same caverns at a point farther from the Dragon's own location than the stairs. This was all the opportunity she needed.
What she did next was perhaps foolish. But she had to do it at this point. Partially because, despite all her Cosmic-spawned super-equine senses, she felt the atavistic desire to see her foe, to confirm her conclusions.
She crawled, inch by inch, up the turn of the stair.
And she saw it.
The ancient crevice had here been greatly widened, as if some vast creature, massing a hundred tons or more, a living excavation machine with teeth harder than diamonds and claws of iridium steel had torn its way through stone and armor like a foal digging through loose soil. A monster to which the best which the Age of Wonders could build was little more than cardboard and paper-mache, to be ripped aside at whim.
Which was exactly what had happened.
And the monster which had done this was right there.
She saw the Dragon, or rather part of the Dragon, and he was completely blocking the upper exit of the stairway, and would have been even if the destruction he had wrought pushing his forequarters into the launch tube had not almost completely blocked that exit with rubble. The part she saw was a huge, curved plate of what looked like metal in the dim light, but which she could smell was in fact a complex composite of silicon polymers grown into a plate of iridium steel which had been organically-desposited, grown by the great beast of whom this was but one mere lamellar scale, the creature which had lain itself casually and perhaps even unknowing across the stair in the process of draping itself partly out into the launch tube, from which it clearly apprehended the main danger.
She considered attacking. It would be easy. A full-powered gravity lance, emitted at this angle, would penetrate right through even the inches-thick iridium steel of the Dragon's ventral scales, through the immensely-tough metal-reinforced bones underneath, and possibly even out through the dorsal armor above. Swing the beam slightly as she did so, and she would wreak fearsome damage within the creature, tearing apart flesh and pulverizing internal organs. It might be a mortal blow, though she did not know if it would be instantly fatal. Probably not, she admitted to herself. Dragons are tenacious of life. A Dragon this size would have multiple hearts, batteries of lungs, the ability to in emergencies compartment off parts of its circulatory system. Even though it lacked her own rapid regenerative powers, it would be almost as hard to slay as herself.
But the Dragon, caught unawares, would not have its strongest defenses up. It would not be maneuvering to avoid critical hits, and its shields would not be up to spread and blunt her own gravity lances. It would be vulnerable in a way it would never be if it knew she was attacking.
In the end, three things caused her to decide against attacking then and there.
Firstly, Celestia had told her to avoid killing unless unavoidable, and there was no way she could strike like this with enough force to subdue the Dragon and be at all sure of not killing it. Secondly, in either case the Dragon in its agonies would thrash about and could easily bring down the whole cavern system upon their heads -- an event Luna would surely survive, but which might prove fatal to Summer Lightning or the unknown captive. Thirdly, it would be a foul blow, and Luna did not like foul blows. She was a simple Concept, after all, and had an equally-simple sense of honor.
So she backed away again, and in her motion made a small mistake, but one which might have had large consequences. Her hoof brushed a pebble,which bounced down the stair, out of her field of silence, making a small clatter.
A small clatter, but a Dragon's ears are sharp.
Luna froze in horror, afraid to move rapidly and hence make more noise that would pinpoint her location. She could teleport, but that would draw its attention more surely, and then Summer would be left exposed to its wrath. Her best bet would be to put up her own shield, and fight directly -- if the Dragon could sense where she was.
A huge shield spell engaged above her. She saw the armor plate move rapidly across her narrow field of vision as the Dragon shifted position. There was a moment of full visibility and then something which she recognized as part of a great foot blocked the stair. From somewhere came a great indrawing of breath, as the Dragon sampled the air.
"I SMELL YOU!" the Dragon roared. "YOU'RE TRYING TO CLIMB THE SHAFT! WHERE ARE YOU HIDING? NO MATTER!"
Luna felt the air begin to ionize around her and might have acted with a shield spell of her own, which would have given the game away, had she not realized something very important.
The Dragon thought they were trying to climb the shaft. That meant he did not know they were on the stair. His head and upper body were in the shaft. He was about to breathe lightning in the wrong direction.
They were safe, as long as they kept calm.
Summer, don't shy, she thought urgently, but with no attempt at telepathy as that too might have drawn the Dragon's attention. Keep calm. It was more of a fervent wish than an attempt at communication.
Blinding light flared from above. Thunder roared, almost deafening, even muffled by rock and armor and the Dragon's own massive form, as nigh-solid ionic bolts screamed down the launch tube. The walls of the stairwell shook, and dust and small pebbles rolled down the stairs. And this was just from proximity to the Dragon's breath. Nothing not shielded by strong magic could have survived a direct hit, and even anypony caught in the fringe would have been slain. The sound persisted long enough that she knew that the Dragon was angling his blast slightly, scouring the walls of the shaft to ensure catching any attempting to cling to them would be struck.
When the world stopped shaking, she began picking her way carefully -- very carefully, back down around the curve of the stair. She kept an eye on the Dragon, saw him move, readied to raise her shields the moment she saw his eye or sensed any sign that he was preparing to breathe down the stair -- even his deflected blast would be enough to hurt her, where she was right now, and possibly harm Summer, especially as it would certainly trigger further collapses.
She saw more ventral scales, then one settled down on the gap. The Dragon had still not realized they were on the staircase! Possibly, it had not even noticed the staircase, or realized that it was still open most of the way down to beings as small as Ponies. This made it even more obvious that he had only taken up residence here very recently -- any Dragon who had laired here long would know every nook and cranny of his lair, and have taken steps to prevent ingress even from passages too small to admit his own body.
In either case, the Dragon had made a mistake, and one which they could still exploit.
She made her way back to Summer Lightning. The small Pegasus was still where Luna had left her, standing firm. She was trembling slightly, but had plainly not panicked.
Luna leaned in close and formed a hollow bubble of silence around their heads.
"The Dragon's blast was aimed down the launch tube," Luna said softly. "He knows not we are upon this stair. Halfway up to him is a fissure that leads into his caverns. By this way shall we pass him."
"Yes, Ma'am," replied Summer.
"As I said, when we find the captive thou shalt see her to safety. I shall wait behind, to bar the way against the Dragon."
Summer said nothing, but her face showed a protest that both gladdened and troubled the heart of the Alicorn.
"Thy desire to guard mine own self says well of thee, but thou canst not survive the struggle against such a drake in close quarters, as can I," Luna pointed out. "And the captive may sorely need thy aid. T'is no shame to do thy own part in this quest."
"Yes, Ma'am," Summer nodded. "I understand."
"Then fare us well -- to the rescue!"