In Town Hall, Twilight quickly took control of the situation, making sure emergency supplies were still viable and equally distributed amongst those hiding from the vines. Spotting one area Engineer had been unable to guard against, she cast her strongest barrier spell over the ground itself to keep the vines from coming up through the floor. However, keeping the barrier up was rapidly draining her magical stamina.
Spy was the first to notice this. "Twilight," he suggested, "would it not work better to anchor the spell, rather than trying to keep it up continuously?"
Twilight smiled ruefully. "If I had anything to anchor it with, I would. But I don't have any of the tools I would actually need to make anchors-"
"What about the sentry turrets and dispensers?" Spy offered. "They are powered by Australium, which is the same as Mana Stone in this world. Would that not be an ideal anchor?" He showed her the small chunk of Australium that powered his cloaking device to demonstrate.
Twilight looked it over as best she could, unable to focus on that while also maintaining the spell. "Looks about right...I can give it a try..." She refocused her magic, seeking the chunks of what to her was Mana Stone with which to anchor the spell. Surprisingly enough, the spell anchored easily, allowing her to sever her link to the working and conserve her strength. "Thanks for the suggestion," she admitted.
"Someone needs to manage the civilians," Spy replied with a shrug. "I am not much for siege warfare when my opponents are non-sapients like what we face here."
Twilight glanced around, looking for some way to utilize Spy's skills. "Think you could keep the fillies and colts entertained?" she requested, gesturing to the large group of underage ponies. "I'd rather they not get up to any trouble or fall prey to the vines, and you must be good with foals if you raised Fluttershy, right?"
Spy chuckled. "Well, I can try." He walked over to the younger ponies, drawing his knives. "Hey kids, want to see a magic trick?"
"Engy hasn't got the respawner working yet!" Scout called out to him.
Spy rolled his eyes. "I know that," he scolded. "I wouldn't show that trick to them, anyway!" He then proceeded to dance his knives around his fingers, much to the amazement of the young ponies.
Smiling, Twilight walked over to Fluttershy. "Think they can handle...whatever this all is?"
Fluttershy nodded happily, turning out a window to fire another blast of weedkiller from her modified flamethrower at another growth of vines. "There's nothing here they can't handle, I'm certain."
A large explosion broke Fluttershy's concentration momentarily. "Gotcha!" Demoman called out, having exploded yet another vine cluster that had evaded the acid turrets.
"...I think I can honestly believe that," Twilight muttered, glancing out towards the Everfree.
Deep in the Everfree, Pyro led the way through the attacking vines. Each one received a touch of his magic flames, burning them to a crisp to the roots. Soldier, Heavy, and Engineer walked behind him, spread out in defensive posture, with Medic behind them. They focused on presenting a capable enough front to convince most of the creatures of the Everfree to not mess with them. For the most part, this worked well, especially after Heavy wrestled a crocodile that seemed to be made of rocks into submission and uppercut a wolf made of wood into a hydra's mouth. The hydra had left, apparently to get the taste of wolf wood out of its mouth.
As they progressed, after each vine Pyro shook his hands a bit. While the magic fire came easily to him now that he knew what it was - and gotten some instruction from Twilight in how to control it - the more he used it, the more his hands hurt. They felt unusually hot, and the joints protested flexing. It got worse each time he channeled the fire. However, since it didn't hinder his ability at all, he ignored it. Besides, without Fluttershy there to interpret - it had been a surprise to him that she could understand him - he had no way to make Medic understand what his problem was.
Following the path of greatest resistance, they eventually found their way to a wide gorge before a massive castle. Deep within the gorge, a faint light could be seen from within a cave. Without hesitation, the group leapt downwards into the gorge, the drop being nothing any of them hadn't done before.
Entering the cave, they saw a massive crystal tree wrapped in the vines, along with two large bundles of the vines.
"Engy!" Soldier barked. "What do you make of this situation?"
Engineer stepped forward, pulling out one of his scanners to analyze the situation. "The tree is high density Mana Stone," he explained. "Super high density. More concentrated than anything I've ever encountered in Australium. It's...actually become concentrated to the point it's alive."
"Why vines hurt tree?" Heavy asked curiously, trying to pull one of the vines off the tree.
"The tree seems to be emitting some sort of wavelength that is inimical to the vines' growth," Engineer explained. "If I had to guess, I'd say the vines have been trying to grow for a long time, but the tree was holding them back...but it's run out of power." He scanned the two large bundles of vines. "Also, Princess Luna is confined in the right bundle."
Nodding, Pyro stepped forward and seized one of the vines in the bundle. Sending his magic fire into it, he burned away the vines. However, as the last of the vines fell away from Luna in cinders, he held his hands together, letting out a deep grunt of pain. His hands smoked, and the latex of his gloves were melting.
Princess Luna was quick to take charge in regard to Pyro. "Do not use anymore fire just yet," she instructed. "I know not why it is causing you pain, but it shouldn't. When this situation is dealt with, we will seek out Twilight Sparkle. She is most adept at analyzing magic, and should be able to deduce what is happening with your magic."
"What would you have us do in the meantime, Princess?" Soldier asked, saluting.
Luna smiled. "For now, General, I believe we should attempt to free my sister from that other pile." She gestured slightly. "At present, mine own magic is greatly diminished from being held so long, so I will be of little practical aid. Failing that, finding some way of reenergizing the Tree of Harmony should aid us."
"The what?" Heavy asked in confusion.
Luna gestured to the tree. "The Tree of Harmony. It is the source of the Elements of Harmony." She noticed their blank stares. "The...necklace Fluttershy wears, and the matching ones worn by her friends, and Twilight's crown?" More blank stares. At this point, she realized it wasn't so much not understanding what she was saying, but the significance of her words. She thought for a bit to find a way to explain the power of the Elements of Harmony to her Team. "...Orbital Friendship Cannon?" she finally offered.
"Oh!" Soldier said in understanding as the others nodded. "That rainbow light thing from the stained glass windows up at the big castle?" He pointed towards Canterlot.
"Yes, that," Luna confirmed, somewhat deadpanned. Her mouth felt dirty for simplifying Equestria's greatest relics in such a manner. "So it is important to find a way to reenergize the Tree."
"My hard hatted friend," Medic spoke up suddenly. "You said the Tree was made of high density Mana Stone? To the point of becoming alive?"
"Yes, at least as far as I can scan," Engineer admitted.
"Then energy of the same kind could presumably reenergize it, or at least provide what it needs to energize itself?" Medic continued.
"Seems logical," Engineer admitted.
"What are you planning?" Luna inquired.
"Well, my Medi-gun runs on Australium, so..." He stepped forward with a manic grin on his face. "It's time to practice medicine!" He switched the Medi-gun to Uber-charge, pointing it right at the tree.
Luna stared in bemusement. At first, she found the entire notion absurd. It couldn't possibly be that simple to restore the ancient source of Harmony, could it? And it didn't look like it was working, as the tree remained as it was as the Medi-gun slowly began to lose power. However, as Engineer began taking chunks of Australium from their other equipment and plugging it into the Medi-gun to give it more power, the tree began to regain its glow, and the vines began to unravel and dissolve.
"One...more...push!" Medic proclaimed. Connecting all the Australium they had with them to the Medi-gun, he cranked it to a new setting a blue-maned unicorn in Ponyville had suggested, one which Engineer had added to all their weapons and gear.
Up to eleven.
A massive discharge of all the energy the gun had in it - enough to make it explode - blasted into the Tree, restoring it to its full health as it released a wave of power that blasted the vines away. Celestia was released. She looked up, blinking in confusion. "Where is Twilight?" she asked.
"Back in Ponyville, managing the civilians," Engineer explained. "She shows signs of being a capable administrator."
Celestia looked very nonplussed. "And...why did she remain there?"
"Because under standing orders from Princess Luna regarding her unexpected disappearance, we were handling the situation!" Soldier barked out.
Celestia turned to give Luna the fish eye, only to receive a blown raspberry in response. She sighed. This may cause problems in the future, she thought to herself. Glancing back, she saw that the Tree seemed completely content with being reenergized, and showed no sign of giving anything back. "And the Elements?"
"With their Bearers," Engineer replied.
"...then we should rejoin them," Celestia murmured, rising to her hooves.
While Ponyville celebrated its liberation from yet another threat, Twilight examined Pyro's magic in minute detail. "Well, I've figured out what's happening with your magic," she said at last.
Pyro sat back to listen eagerly, his hands now feeling much better.
"Your magic isn't raw mystic energy like it is with most ponies," Twilight explained. "I don't know if it's because you're human, because of your high exposure to magical radiation from the Australium, the apparent frequent respawning, or because you're - well - you. But it's already focused inside you...as mystic flames."
Pyro nodded. He had always felt the flames inside.
"It's not healthy for a living being to contain magical fire," she explained. "Even worse to release it as you have been. If you keep it to small amounts, you should be fine. But push too hard, and you'll suffer similar instances to what you experienced today, where your hands start to burn and melt. Push beyond that, and it'll expand to beyond your hands. Your body generates the magical flames faster than you can use them this way, though they seem safely contained inside you when you aren't using them."
Pyro waited for more explanations.
"However..." Twilight shivered. "If releasing the flames does enough damage to your body without being repaired, if you push the flames too far...they'll all come out at once, and you'll go up like a miniature supernova, destroying everything in your vicinity. And I don't know when - or if - the flames would stop."
Pyro stared at his hands, letting this knowledge sink into his mind. Eventually, he nodded.
"That's it?" Twilight asked, surprised. "I tell you that you could end up destroying all life on the planet if you push your flames too far, and you just...nod?"
Pyro gave her a thumbs up. After all, for the longest time, he thought he was destined to be nothing but a monster that would bring fire and destruction to all that lived. As he saw it, Twilight had just told him he had a choice in the matter, control of when - and if - it happened. As far as he was concerned, that was an improvement.
Twilight threw up her hooves with a groan of frustration. "...men!" she finally spat out, causing Pyro to let out a belly laugh.