The Doctor took a deep breath and frowned. Absentmindedly, his tail brushed off an old, dusty, cobwebbed chair so he could sit down in it, one hoof coming up to tap at his chin thoughtfully.
"I have read about them... once. I wish I had the book, it's back at my personal library in Ponyville. Ah, what was it? Menaces of Magic? No... Profane Powers Beyond Ponies? No, not the Poninomicon... The Bestiary of Banal Threats? Perhaps. Hrm. Well, there's definitely a good news, bad news thing going on here. Good news is—your guards are still alive. That's why the Manasprites have been stealing food—to feed the guards. They've got them webbed up and cocooned, and are feeding off their unicorn magic. Bad news is, that means they'll just keep multiplying as long as they have your guards."
Luna perked up at the Doctor's "good news." Those poor guards. she thought to herself. How long had they gone trapped down here, unnoticed? How many of them were there? Even though Doctor Whooves had said they were alive, that was only as far as the Manasprites were concerned. There was no guarantee that the guards had been able to survive without sunlight, or that they hadn't just lost all hope and starved themselves. And what of illnesses that could be contracted in place like this? It made Luna sick to think of them trapped down here, not knowing their fate, and possibly dying a slow tortured death. She swallowed a lump in her throat, not even attempting a brave face.
The Doctor’s explanation brought Celestia much needed respite. She breathed easier knowing that the Manasprites were not malicious; for a brief moment she feared that they would attack and consume magic-using creatures with the fury of a hungry Manticore, but the relationship was less predatory and more mutualistic.
Luna paused in her worried trot, then turned and made a beeline for Celestia. She stood next to her sister, laying her head on her sister's shoulder and then looking up at her. "T—Tia—" Luna cleared her throat, silently excusing herself for using the childish nickname. "Celestia... what'll we do?" Her heart was still pounding, anxious of the path they'd left behind—and the uncertain path that lay ahead.
Celestia’s countenance softened at Luna’s stuttered response. Eons of experience did nothing to dampen the raw, visceral, fear of the unknown, nor the unnerving nature of many magical creatures’ shadow in the aether. A faint smile tugged at her lips at the mention of her diminutive name from a time long past.
"We’ll do as we’ve always done, Luna," the sun princess replied, gently nuzzling Luna’s cheek. "Make our way through the chaos together."
The Doctor pondered a bit more, then lit up. "Ah! I remember something!" He trotted quickly over to the door and swung it wide open. The entire corridor was awash with surprised-looking Manasprites, and the door itself was coated with them, too, like a multicolored fuzzy rug. The Doctor casually grabbed one bright blue Manasprite off the door and closed it before they could react.
The sound of creaking hinges made Celestia’s ears perk up. The princess whirled around in time to see the brown earth pony crack open the door. Her horn glowed faint lavender again as she prepared to stem an oncoming tide and rebuke a foolish—SLAM! The door was closed just as quickly as it was opened. In the Doctor’s hoof was a single Manasprite, looking just as confused as Celestia.
"They're terribly guarded against magic—attacking them directly with spells will just make them multiply, make them stronger. But..." He dropped it to the floor and dropped his hoof onto it, smashing the thing against the ground. Rather than the crunch of an insect meeting its demise, there was a loud *pop* as it burst in a shower of blue smoke and red sparks, washing up over his face and making his mane stand up a bit, his face getting just a little sooty. "A little impact will disperse them back to the magic they're made of. But the bad news is, if you do that to all of them at once... it's kinda explosive, unless you have some way to contain or absorb all that magic."
Celestia’s incorporeal senses tingled as the fountain of sparks slowly subsided. While the lone Manasprite was a mere pinprick of void in the aether, the sparks from its disintegration made her hair stand on end. The aether had palpably changed, becoming charged with energy very suddenly with just the tiny explosion.
Luna was caught off guard when Whooves opened the door they had come from, and she shirked into a dark area of the room in an attempt to hide from the inevitable onslaught. But it never came, and Luna was surprised at how easily a Manasprite was exterminated. Of course, it would take an army of ponies to make a dent in them, since magic had no effect.
"This is dangerous," Celestia said, "An explosion of Manasprites would not only cause physical damage, but have serious repercussions in the aether as well."
As the doctor went on, Celestia continued to pace, studying the heavy iron-studded doors of the conference chamber with quiet curiosity. Magic offered no purchase against the Manasprites, but wood and stone and iron proved quite the impediment. The dungeons were honeycombed with these passages, built for transit and housing in the event of war or some other disaster. The oldest structures probably dated back to the time of Discord. Somewhere in these passages, the source of the Manasprite infestation lay.
The Doctor let out a frustrated noise. "They're not supposed to exist anymore! This is supposed to be impossible! Ahhh, well. Four stars, bonds loosed. I should have expected anything." He glanced up at Princess Luna. "AH! Princess, your magic was always the strongest, back in the day. Celestia couldn't hold a candle to your might! Have you recovered fully from the Elements of Harmony yet?" Then he frowned. "No, no—if you had, your mane would be all sparkly, like the beautiful night. Well, at least that means you're safer from the Manasprites. They'll all head straight for Celestia."