AN: Thank you Nox Zephyrus, Goku275, fartmaster1002, Wadeparker, and MDM11.
fartmaster1002 (like the name, by the way), thanks for the suggestion, though I don't think I'm going to use it. I will say, however, say more about Natsu's flames in the AN at the end of the chapter.
As soon as August and Natsu appeared in an empty plain a few miles from Zeref, the dragon swung his flaming fist at the Spriggan. Unable to replicate (and therefore nullify) a power great enough to overcome the Curse of Ankhseram with his magic, August was forced to avoid the strike by teleporting to a safe distance.
"I thought my brother told you to stay out of our fight," said Natsu angrily. His Magic Power weakened as the tattoo symbolizing Igneel's power dissolved, indicating to August that the threat to Zeref's life had passed.
"He did," acknowledged the Spriggan, tightening the grip he had on his staff. "But…"
August's actions went directly against Zeref's orders. He noticed that his father had accepted — no, welcomed — his imminent death with open arms. And yet…
"…As a child of the emperor, I could not simply stand back and watch him die, regardless of the circumstances."
August, to his surprise, saw a look of understanding and respect form in the Fairy's eyes.
"Yeah, I suppose that I've done that sort of thing before," admitted Natsu. "It's only natural to protect those you care about, even from their own self-sacrificing choices. However, that doesn't mean I'm going to give up. I'll just defeat you here, then find another way to take my brother down!"
"It is not possible for you to defeat me," responded the Spriggan. His appearance transformed, his skin changing color and developing golden lines as he prepared to fight. He wouldn't allow a threat to his father's plans to continue living.
"'Not possible'?" echoed Natsu rhetorically. Bright red scales spread across his body, and wings and a tail sprouted from his back. August felt his body tense up in the face of the Fairy's Pseudo-Dragon Force. "I'll take that impossibility and burn it down!"
"Fire Dragon King's Destruction Fist!" shouted Natsu, throwing a punch at August. Even with Igneel's power gone, his flames burned hot enough to make the ground dry up and crack. As a result, the dragon was surprised when his attack struck the Spriggan yet didn't burn him.
August took advantage of this, channeling Magic Power into his staff and swinging it at the Fairy. Natsu endured as it smashed into his left arm, then threw a kick at his foe.
"Fire Dragon King's Claw!"
Again, the attack landed (hitting August's ribs) but didn't leave a mark. This time, though, Natsu was able to jump back in time to avoid the Spriggan's counterattack.
"What's going on?" wondered the Fairy. It had been years since he faced an opponent this resistant to his flames. "Is he a Fire Dragon or Fire God Slayer? A Fire Devil Slayer? No, he'd be using his own flames if that were the case. Could it be… is he just that strong!?"
A shiver of fear ran through the dragon, only for him to shake it off a moment later.
"It doesn't matter!" shouted Natsu to himself. "If my flames aren't hot enough to burn you, then I'll just burn even hotter! Fire Dragon King's Brilliant Flame!"
The Fairy brought his hands together and unleashed a massive torrent of fire from his palms. Unlike before, August responded to this attack by bringing his arms up in defense instead of just letting the spell hit. And, when the flames died down, the Spriggan's clothes were singed.
"Now take this!" roared Natsu, emboldened by his success. "Fire Dragon King's Claw! Fire Dragon King's Destruction Fist! Fire Dragon King's Sword Horn!"
The dragon threw a barrage of attacks at August, kicking, punching, and even headbutting the Spriggan. Scrapes and light burns began to dot August's body, forcing him to escape by teleporting a few yards away.
"His strength is absurd!" thought August, irritated.
While he had initially been able to use his magic to copy and nullify Natsu's flames, the dragon's power had only continued to grow as the fight went on. The fire had already surpassed the Spriggan's ability to cancel out, something that only Zeref and a serious Irene had been able to accomplish in the past.
"It seems that conserving my strength for a potential clash with Acnologia was a mistake," concluded August. "If I want to defeat Natsu, I must use my full power!"
"Fire Dragon King's Roar!" bellowed the Fairy, an inferno erupting from his mouth and shooting at the Spriggan.
As the flames approached August, he flooded his staff with Magic Power and swung it downward like a sword. The storm of fire was split in two, ravaging the land around the Spriggan but leaving him unscathed.
Natsu stared at the scene in shock, giving August the opportunity to go on the offensive. Imitating the magic of Invel, he froze the Fairy whole. Then, as the ice was reduced to water vapor by the dragon's flames, the Spriggan closed the distance between the two of them and unleashed a massive explosion at point-blank range.
Natsu was sent flying backward. August chased after him, swinging his staff down on the dragon with enough force that a crater formed when he hit the ground. The Spriggan prepared to perform another attack, only to retreat when an intense flame surged forth from the Fairy's body.
Natsu rose to his feet, the damage that his clothing had received during their battle exposing scales across his arms, legs, and torso.
"So you were also holding back a bit," spoke the Fairy, a fearless grin on his face. His body started to grow, the scales spreading as he entered his true draconic form. "It looks like neither of can afford to do that, though. Man, I'm all fired up!"
"Fire Dragon King's Roar!" bellowed the dragon, bathing the area in front of him in flames. August raised him staff in front of himself and formed a barrier, opting to endure the spell rather than risk greater harm by attempting to dodge or counterattack.
When the flames faded, the Spriggan dropped his barrier and bolted toward the Fairy. They both swung their right arms, and luminous staff met blazing claw in a collision that produced shockwaves.
Neither of them was deterred, though. Over a dozen more strikes were thrown before the exchange ended, and both of them came away from it with a few new injuries.
With the clash over and nobody holding a clear upper hand, the two of them put some distance between one another. August leapt back, while Natsu took to the sky. A few seconds passed, then the next attack commenced.
"Fire Dragon King's Sword Horn!" shouted Natsu, swooping down and folding in his wings to ram into the Spriggan at full speed. The Fairy saw his foe's eyes widen in surprise at the sight of a massive, flaming dragon shoot at him like a rocket.
In fact, August was so caught off guard that he was barely able to avoid a direct hit. Even then, his left arm was clipped by one of Natsu's folded up wing, leaving it gouged and burned. The Spriggan recovered from his shock quickly, though, and fired an intense beam of energy at the dragon's underbelly that left a wound as large as the one August had just received.
Of course, on the huge body of a dragon, such a wound wasn't as deep. As a result, the Fairy was able to twist back in his enemy's direction and perform his next spell without being slowed down.
"Fire Dragon King's Crushing Fang!" yelled Natsu, swiping his flaming claw in a downward arc. August leapt back but was unable to dodge completely. Three long, shallow cuts marked the Spriggan's chest, bordered by second degree burns.
"I have dedicated my mind, body, and soul to His Majesty!" exclaimed August, his determination unshaken by his injuries. "I will not be defeated by you!"
With that, the Spriggan unleashed a beam of energy the size of his body. As it approached Natsu, the Fairy could tell that even his dragon form would be seriously damaged by a spell of this strength.
"Dragon Slayer's Secret Art…" started Natsu, his right arm igniting. The Fairy fearlessly shot toward the Spriggan, prepared to meet his attack head-on. "…Crimson Lotus: Exploding Flame Blade!"
The dragon cut through the worst of August's spell, reducing the damage he received as he closed the distance between the two of them and hit the Spriggan dead-on. Having committed to his attack, August was unable to avoid or mitigate this strike and was sent flying, his body battered and badly burned.
"You might have devoted everything to your emperor, but I'm fighting for my family and my guild, making me invincible!"
The love between a parent and a child.
Despite having lived for decades… despite being the greatest of the Alvarez Empire's Spriggan 12… August had never been able to grasp this concept.
A child loves their parents, and a parent loves their children.
Yet, the emperor — his father — never loved him. Why did the emperor never love his child? Because he never realized he had one.
…
…
…
'I'm fighting for my family.' It was these words that enraged August the most. Natsu was a child loved by his father and a parent who loved his children. He fought with gifts bestowed upon him by the former for the sake of the latter. He both received what August desired and freely gave it to others.
"Even if it costs me my life…" decided the Spriggan. "Crumble and turn to dust! Thus do I offer up my life!"
As he began chanting Ars Magia, Natsu started coughing. The spell evaporated the blood of living things and melted the very land. Fueled by August's vast Magic Power, it was enough to kill even a dragon.
"Begone!" August ordered the hacking Fairy. Casting the spell might kill the Spriggan, but it would be worth it to see Natsu die.
"I… already told you…" replied the dragon through gritted teeth, struggling to his feet. "My love for… all of them… makes me invincible!"
As if to punctuate his words, flames ignited across the Fairy's body.
"Impossible!" thought August. "Ars Magia should be preventing him from using magic!"
"Dragon Slayer's Secret Art," spoke Natsu. The flames gather around his fist, growing hotter and hotter with each passing moment. They didn't stop until they were as powerful as the fire that had almost killed Zeref. "Purgatory Slashing Dragon Fire!"
Dread filled the Spriggan's soul as the dragon punched through his chest and burned away his most dangerous and powerful spell. In spite of August's best efforts, his father's life was still in danger. Yet, oddly, he also felt a bit of warmth.
"You definitely lived up to your reputation as the strongest of the Spriggan 12," said Natsu. The dragon, while currently standing over his dying opponent, wasn't in much better shape himself. Although his life wasn't in danger, his body had been ravaged by the battle and his Magic Power was running low.
"Thank you… Uncle," replied August, surprising the Fairy.
"Uncle? Is the blood loss getting to you?"
"No… my mind in working," answered the Spriggan. This made Natsu even more confused. "You see… my father is… your brother Zeref… and my mother… Mavis Vermillion… though neither knew… of my existence…"
Natsu's jaw dropped. What August was saying was unbelievable, but the Fairy could tell that every word of it was true.
"Do you think…" continued August. "If they knew about me… then… would they have loved me? Would they have… held me in their arms…?"
The dragon bent down to the Spriggan, shifting back to his human form. He wrapped his arms around his nephew, giving the dying man a hug.
"Of course," replied Natsu softly, tears rolling down his cheeks. "Because there isn't a parent in the world who doesn't love their kids. And I'll tell them about you, so that they can both hold you when you all meet up in the afterlife. I promise."
"That would… be nice… thank… you…"
As August spoke his final words, he shed tears of joy at his uncle's words.
Five minutes later, Happy arrived (having located his partner by the destruction that the fight caused) to find a badly injured Natsu kneeling before a simple, freshly-made grave. The Exceed paid his respects to the grave, then began flying the dragon back to the guild hall. On the way, Natsu explained what had transpired since their separation.
Omake
As Happy flew the now-resting Natsu back to the guild hall, he thought about what the dragon had told him.
"Having a son you never knew about, who was also an enemy and is now dead…" said the Exceed in a subdued voice. "I feel sorry for Natsu, promising to tell the First Master that."
It was at that moment that Happy realized something important.
"Wait a second! This the sort of really important thing that we have to tell the First Master about right away! If Natsu's out cold, doesn't that mean that I have to tell her in his place!?"
AN: Now, to be clear, Natsu's flames reached Igneel's level for the final attack and that attack only. Once he recovers, Natsu's flames will be stronger than before he used Igneel's power but still not yet at the level of his father. This surge was due to the stock trope of powerful opponents and strong emotions bringing out a character's full potential.
Also, the name of the technique he used came from chapter 62 of 100 Year Quest. I won't give any spoilers, but I will say that it felt fitting given what was said just before canon Natsu used the technique.
I hope you all enjoyed this chapter. Goodbye!