Direct. Condense. Time with the moment of contact. Explode. The concept of Shunpo was to use collect and solidify Reishi beneath your feet, while sending a pulse of Reiryoku down the foot at the moment before movement in a burst to amplify the movement's explosiveness with a thin film of Reiryoku to shield the body (though unnecessary for those stronger) and help negate resistances to the speedy movement.
However, Natsu did not have the body to do such a technique, lest his muscles tear and bones break under the sudden pulse of energy and pressure his ankles were not ready to handle… yet. So, he improvised. "This is no Shunpo, but it will have to do for now."
Flames burst beneath his feet upon contact with stone with precision, exploding out in one direction in a controlled burst to accelerate Natsu into fiery blur, leaving trails of fire on the stone floor. Despite his last two victories, Natsu's magic reserves were still around half its capacity, and constantly dropping from his improvised use of Shunpo, which he named Flash Fire as an afterthought, and his Reiryoku reserves drained it even more in an attempt to replenish itself.
A scent and shift in the air was the only warning Natsu got before the area in front of him exploded.
"How sad.."
A man appears in a gust of wind. Eyes blindfolded with tears streaming down his face, the man settles down away from Natsu. The cross necklace along with the embroidered cloth around his neck swayed in the wind as he settled into a stance.
"I am Aria, the Pinnacle of the Elemental Four."
The winds around them grew stronger, making Natsu's own scaly scarf flap around.
"And I.. have come to slay the Dragon."
Natsu let out a snort, gently patting Happy who popped his head out from the scarf Igneel gifted him. He could sense the magic Aria was emitting, how it permeated the air around them, molding the magic to ready an attack. But despite feeling the clear difference in magical power, Natsu failed to feel an ounce of trepidation.
"Let me show you my magic!" Aria thrust his palm out.
The air rippled. Natsu dashed to the side in time to avoid getting crushed by the invisible attack and rushed forward. The air rippled once more, disturbances noticed by Natsu's highly tuned senses that let him dodge the air spaces with almost precognitive speed.
"Fire Dragon's…"
Forgive me old friend, I'll have to bastardize your technique for now, Natsu lamented. In response, Ryujin Jakka pulsed in understanding and what may have been an undertone of annoyance. Flames that spouted off at Natsu's fist coalesced onto the edge of the blade, a fine sheen of pure heat that split the air clean as it was swung.
"... Sun Seared Edge!"
The Zanpakuto clad in the Fire Dragon's flames sliced cleanly through the airspace, as if the magic was never there. The heat became palpable, raising the temperature in the room rapidly despite there being no flames seen.
Aria was in shock, whatever data Phantom Lord had gathered on Natsu failed to mention anything about his current skills displayed, nor about his sword. Moreso, he was shocked by the fact his magic was broken through so easily. Whatever cold sweat that had supposed to break out by now was instantly evaporated, the stifling waves of heat visibly distorting the air around Natsu's blade, steadily raising the temperature around them.
"Im-Impossible! You, you weren't supposed to be this strong." Aria shouted out in fear, throwing out spells one after the other with his blindfold taken off. With each spell thrown, he took a step backwards.
It proved futile. Each air space was cut through, a flameless blade burning away the magic and disrupting the air forming it, streaks of flame decorated the place as Natsu matched Arias step backwards with a Flash Fire forward, quickly closing the gap.
"FALL UNDER THE AIRSPACE OF DEATH!"
Winds swirled around Aria, a whirlwind of Airspace magic with countless spheres, all aimed to kill a certain Dragon Slayer. The sudden air pressure locked Natsu in place, even his magic could not break him out of the hold easily. Aria's eyes glowed with malice, a maddened grin etched onto his face as he gloated in self assured victory.
"ZERO!"
A visible wall of wind, a veritable tornado of Airspace magic rushed towards Natsu filled with intent to kill.
With a grunt, Natsu brought out more of his magic than he was willing to use from the fight, breaking the hold Aria's airspace had on him. He glared at the wall of Airspace magic rushing towards him.
And so, he mused.
"Breaking it with brute force would be counterproductive. Using my magic would put my reserves in the red. My Reiryoku hasn't recovered yet to pull off something big after using Danku, nor is my body trained for the Genryu sword style to be used." Natsu narrowed his eyes at a certain part of the Airspace magic. A weak point was discovered, only found because of Yamamoto's millenia of experience and the Dragon Slayer's sharp eyesight.
"Hm.. That will do."
Brought forth by his reluctance to waste energy and prior experience, the Yamamoto in Natsu noticed a flaw in the magic, a centre point holding it all together. And with nothing else at his disposal without large risk, he turned back to where his skill started. The basics. A stance, knees bent and breath steady. Both hands grasped his sword, drawn to the side as he readied to cut the blade down. And then, he swung.
Mind over matter, a discipline forged from more than a thousand years of constant, diligent and bone breaking practice let Natsu follow the movement with perfection and no wasted energy, despite his body not being used to it. A simple swing forged from the basics, aimed to sever the magic before him, nothing more, nothing less.
"Taimatsu."
Aside from that little flair that is.
The flames trapped on the edge of the Zanpakuto roared. Empowered by the swing, the pressurized fire engulfed the locus of the spell, breaking it down into a whirlwind of fire and ashes. The room quaked from the sudden explosion, breaking the wall as a gout of fire made it's way outside.
Aria erected a barrier of Airspace magic before him, but the heat still got to him as his lip cracked and throat dried. He coughed as the fire before him raged, smoke and ashes filling the air gradually.
"..Ha...Ahaha.. AHHAHAHAHA!"
"LET TODAY BE KNOWN AS THE DAY THE DRAGON DIED, SORROWFUL! SORROWFUL IND-"
"You talk too much."
Interrupting Aria's maniacal laughing spree, Natsu shot out of the inferno, embers and ash clinging to his form and swung his blade once again. Blood flew into the air, the shallow cut stunning the Airspace Mage. Natsu brought the blade upwards, and slammed the pommel into Aria's temple. He crumpled down onto the floor.
'Can't exactly have you dying on me," Natsu grumbled. He punted the unconscious mage through an open door, throwing across a few rooms into a hall, far away from the inferno blazing in the room he was in.
Fatigue washed over Natsu, making him grimace while his Zanpakuto resealed itself in wood once again. The leakage of Reiatsu now plugged, Natsu trudged forward keeping his balance as the building shook again. Unknown to him, the Abyss Break Spell was broken, the arms of the giant structure broke under the strain of its own weight, magic no longer fueling it. The Fairy Tail mages outside cheered, but Natsu had more pressing concerns. Finding Lucy and Mirajane.
But as much as he wanted to run again, his ankles ached from prolonged use of Flash Fire, so he stuck to walking around at a hurried pace, his sealed Zanpakuto acting as a cane.
"Natsu!"
He knew that voice.
"Erza?"
The first thing he saw was red. Not her hair, but blood, both fresh and flowing down her arm and dried blood caking her skin, a dead give away to the injuries she got from blocking the Jupiter Cannon, and some new as she made her way into Phantom Lord's Guild Hall.. Erza had Re-quipped to her standard armour and was running up to Natsu, the metal plates clunking noisily with each step.
A frown was etched onto his face before he realised it, the concerns of both Natsu and Yamamoto melding together into a heartfelt worry.
"You should not be here, you're still hurt."
His cane thumped against the floor resolutely. He set a firm gaze on Erza, stopping her in her tracks. Erza on the other hand, felt a pressure unlike before, not magical in nature but something much different. To her intrigue and confusion, she looked Natsu right in the eyes and recoiled.
It felt as if Master Makarov was in front of her, looking right through her.
"Go back Erza. I'll take care of Jose. His elites have already been defeated
"But the Eleme… Wait, you managed to defeat Aria?' Erza asked with no hidden shock
Natsu turned back with a raised eyebrow and continued to walk forward with Erza hot on his tail.
"Yes, I did. Is that so hard to believe?"
Flabbergasted, Erza did not deign it worthy to let her jaw be picked up from the floor for a good minute. When she snapped out of it, a blush covered her face in embarrassment as she coughed into an armoured fist.
"Forgive me for saying this Natsu, but I did not know you recovered your magic fully, much less got this strong."
Natsu shook his head.
"My magic is still recovering, and I have always been this strong."
A pause. Natsu tilted his head in thought.
"I just… remembered something I forgot a long time ago."
THAT caught Erza's attention.
"You… You beat Aria in that state? Just how di-"
His nose twitched as he picked up a metallic scent. An oddly familiar. The smell of a dragon. His face made to twist into a snarl, only to stop and turn into a raged grimace. This was the same smell those iron pillars had.
Erza got interrupted by the sound of static. Jose's voice echoed through the destroyed hallways of the crumbling Phantom Lord Guild Hall. What he said stilled Natsu, an inferno of contempt barely restrained.
"Wait, Natsu! You better tell me right now w-"
Flames sparked at the soles of Natsu's feet as he vanished into the hallway in front in a burst of Flash Fire, leaving Erza alone. Her eyebrow twitched, the indignation of being interrupted twice and violent tendencies escalating the urge to beat Natsu made her punch the wall next to her.
"...Just you wait Natsu. I swear I'll get an answer out of you later."