Earlier
Mard Geer stared wide eyed at Natsu as the black flames cloaked around him.
"Hellblaze…Dragon? Hmph no matter." Mard Geer quickly regained his composure as he made a swiping motion with his right hand.
A vortex of thorns erupted from the ground, straight towards Natsu. Natsu jumped up high into the air, with the thorns quickly changing trajectories and following right after him.
"HAAAAHH! Join together, the flames of a dragon and the flames of a demon!" The black flames that cloaked natsu began to disappear. Natsu's right hand formed a huge mass of his dragon slayer flames with his left hand forming hellblaze. He immediately converged the two into one as the thorns were right in front of him.
"Brilliant Flame of the Hellblaze Dragon!"
Mard Geer's thorns were instantly turned to ashes, followed by a massive explosion of black and red fire that pushed the demon back.
"W-what is that power that burns away my curse as if it were nothing?!" Mard Geer grunted as he put his hands up in front of him. From the explosion Natsu emerged, his body once again cloaked in Hellblaze.
Suddenly the ground began to shake and a figure burst forth from the rocks below. Natsu recognized the figure as Jiemma Orland, the previous guild master of Sabertooth.
"Let's see how you deal with a demon whose power surpasses even the nine demon gates." Mard Geer said.
"Fairy Tail…" Jiemma grumbled. "Your weakness disgusts me!"
"Out of my way! My beef isn't with you!" Natsu shouted as he wound back his right fist cloaked in black flames and punched Jiemma straight through the chest, causing an eruption of black flames to come out the other end of Jiemma. Jiemma's eyes widened as blood began to drip out of his mouth and he fell to the ground.
Mard Geer stared in shock at Natsu. Something was different about the dragon slayer, not just his power, but his demeanor. His power increase was one thing, but Mard Geer could just get a sense of a violent rage emitting from Natsu.
"MARD GEER!" Natsu roared as he charged straight towards the underworld king.
Mard Geer immediately put his hands up in front of him, causing a swirling mass of curse energy to form in front of him. From the curse energy, an immense wooden beam of twisted roots shot out at Natsu.
"Dea Yggdrasil!"
Natsu swiped his arms in a circular motion, creating waves of black flames that shot out at the wooden beam.
"Dragon Slayer Secret Art! Black Lotus: Exploding Flame Blade!"
The black flames exploded upon coming into contact with the wooden beam, creating a chain explosion of more hellblaze that completely vaporized Mard Geer's curse. Mard Geer immediately jumped up high into the air, floating high in the sky as he looked down at Natsu below.
'How…why is this mere human so powerful…and able to use the flames of both a demon and a dragon?!"
Natsu looked up in the sky and glared at Mard Geer as he began flaring his magic up. Suddenly, the sky became darker and darker, followed by the sound of explosions in the distance. Natsu could no longer see Mard Geer above and Mard Geer could barely make out Natsu's form dimly lit by Hellblaze.
Suddenly Natsu fell to his knees and the black flames around him dissipated, being replaced by a bright yellow light that shot up into the dark sky.
A voice called out in Natsu's head.
'Natsu! The time has come! I trust you will defeat END! I will deal with Acnologia!'
Moments later, the sky began to brighten up once again and the figure of a giant red dragon soared in the air above Natsu.
Mard Geer looked in awe at the red dragon.
"T-thats…"
"Igneel!" Natsu shouted, panting heavily. The red dragon ignored Natsu and looked towards the distance in the direction Acnologia was originally coming from.
"Don't ignore me! Where the hell have you been?!" Natsu screamed. Igneel glanced down at Natsu.
"I don't have time to explain right now. Your friend Meliodas needs my help. However! If you retrieve that book there from Mard Geer, I'll answer all your questions." Igneel replied. Natsu looked up at Igneel and his face lit up with determination. Igneel nodded at Natsu before soaring off into the distance.
"Damn!!!! Igneel, Acnologia and Meliodas! So many interruptions! You will all die once E.N.D Revives." Mard Geer flared as he rushed towards Natsu. Natsu looked back towards Mard Geer and widened his stance.
"Mard Geer! I'm taking that book!" " Natsu screamed as his body became cloaked in black flames once again with dark tinted scales forming across his face and arms.
As Igneel flew towards Acnologia, he looked back as he felt Natsu's magical power drastically rise. 'Those black flames he received from Meliodas. They're quite sinister.'
Natsu took a deep breath in as Mard Geer approached him. Mard Geer's eyes widened as Natsu's already immense magical power increased further. He immediately flew to the side as Natsu opened his mouth.
"Hellblaze Dragon's Roar!"
A black ray of fire and energy shot out of Natsu's mouth. Mard Geer was unable to dodge fully as the attack turned his left arm and left wing into ash. The beam continued straight into the distance carving right through the far away mountains and leaving a trail of black flames in its wake. Natsu wiped his mouth and looked at the trail of destruction he caused.
"Dammit…that's hard to control." Natsu said to himself. He looked towards Mard Geer who was kneeling on the ground in pain, his eyes wide with anger and confusion.
"This…searing pain that destroyed even MY body…" Mard Geer gasped. He glared towards Natsu and slowly stood back up, raising his right hand up at Natsu.
"YOU FILTHY HUMAN! CURSE POWER IS THE GREATEST OF POWER! THERE WILL BE NO MORE MAGIC IN THIS WORLD AND I'LL START THAT BY ERASING YOU, NATSU DRAGNEEL!"
Natsu suddenly found himself surrounded by a thick black mist that creeped closer and closer to him. Natsu felt a wave of fatigue fall over his body as the mist enveloped him completely.
"Now descend into purgatory! This is the ultimate curse created to kill even Zeref! This is…"
The mist around Natsu began to swirl, taking the shape of countless ghost-like effigies that shot straight up into the sky into one massive pillar.
"MEMENTO MORI!"
Mard Geer looked up at the pillar and took a deep breath in.
"In order to murder Zeref, one must eliminate the very concepts of life and death. Those who fall under this curse…will cease to exist entirely! You will be obliterated into-"
"SHUT THE HELL UP!" Natsu's voice echoed. A figure clad in black flames leapt out of the pillar shooting up into the sky. Mard Geer stared at Natsu, dumbfounded by what he was seeing.
"You…should have been erased! Did those flames protect him? Or is it something else…" Mard Geer muttered.
"I'll burn away your future of curses! Because magic is going to take us into the future!" Natsu landed on the ground and immediately leaned forward, putting his hands in front of him on the floor and getting ready to jump forward. The black flames surrounding him began to grow more intense. Mard Geer took a step back as he looked into Natsu's pitch black eyes.
"I'll defeat you with Igneel- no…with both Igneel and Meliodas' power!"
Natsu suddenly vanished from Mard Geer's sight, leaving only a trail of dust and black flames behind him as he suddenly appeared right in front of him.
"Dragon Slayer Secret Art! Hidden Fire Form - Black Lotus: Phoenix Blade!"
Natsu kicked off from the ground, forming the earth beneath him to shatter and leave behind a massive crater. He rammed straight into Mard Geer's chest, pushing the etherious off the ground and into the sky. Mard Geer felt a sharp, searing pain in his body as Natsu pierced straight through him, coming out of the other side of Mard Geer.
"No…" Mard Geer muttered to himself as he glanced at Natsu behind him. He could no longer feel pain in his body as the black flames turned his body into ashes. "It can't…END like this…"