Castle Rooftops.

Natsu and Laxus were about to face Griffon, one of Mundus' demon generals. 

Griffon: Now then, let's see if you can entertain me!

https://youtu.be/hLceKocjDNw

Griffon unleashes a long red lightning beam at them. 

Laxus: Sorry, bird-brain, but your lightning won't- 

Laxus was cut off when the lightning beam surprisely hit him, shocking Natsu. 

Natsu: Laxus!

Laxus: (surprised) The hell? I should've absorbed it, but I can't for some reason. Why?  

Griffon: You seem surprised. As a Lightning Dragon Slayer, you're able to eat and absorb lightning and electricity. However, I wield elements of lightning and wind from the underworld. Meaning my power is too strong for you to handle. 

Natsu's mind: So Laxus can't it demon lightning? It's just like the time couldn't eat Zancrow's flame because he was a God Slayer. 

Griffon: Here I come!

Griffon screeched as he dived forward, beak first. Natsu and Laxus avoided it just in time as the beak pierced the roof. Using this to his advantage, Natsu charged forward.

Natsu: Fire Dragon Talon!

Natsu delivered a flaming kick straight at Griffon's head, forcing the giant demonic bird back. Natsu wasn't through yet as he used fire from his feet as a booster and launched directly at Griffon.

Natsu: Fire Dragon Sword Horn!

Natsu head-butted Griffon, however the demon bird shook it off and smack Natsu down with his wings. Griffon flies to the air with his wings. 

Griffon: You are much stronger than the average human. However, I am far superior to your worthless kind!

Griffon then flapped his wings and sent several large bolts of red lightning down toward Natsu. The Fire Dragon Slayer was able to jump and dodge most of the lightning bolts, but one landed in front of him, knocking him back. 

That was enough for Griffon to launch a rather large bolt of red lighting, which scored a direct hit. Griffon looked down with a smile as the dust settle and Natsu was badly wounded. However Griffon's smile faded when Natsu starts to get back up.

Natsu: Sorry birdbrain, but I got experience with lightning!

Griffon: Amusing. This may indeed prove to be a challenge for once.

He flew down to Natsu and tried to squash him with his large talons; but Natsu caught them with his hands. The Salamander was pushed back a bit. But once he dug his feet into the ground, Natsu slowly stopped and wouldn't move.

Natsu: Think you can outmuscle me? Ain't gonna happen you Dodo-Bird!

He used Griffon's momentum to flip the demon bird over him, sending him rolling across the massive rooftop. When Griffon recovered he saw Natsu in the air just above him. 

Natsu: Fire Dragon Brilliant Flame!

Natsu tossed a massive fireball straight for the surprised demon bird. With no time to react, Griffon flapped his wings and sent an equally large red lightning ball straight at the attack.

Both collided and created a massive explosion of fire and lightning. Griffon soared back in the air from the dust, and Natsu stood his ground. 

 Laxus shot himself upward in a bolt of lightning flew straight toward Griffon's face. 

Laxus: Lightning Dragon Demolition Fist!

Laxus threw out his fist to send a large fist-construct of pure lightning to strike Griffon's face with explosive results before he flew past it. Laxus looked in frustration to see the smoke clear again and Griffon was still undamaged. 

Griffon: Pathetic. Is that all you got? 

Griffon charges his power into one massive burst of lightning strikes inflict massive damage to Natsu and Laxus. The two Dragon Slayers were bruised and injured. 

Laxus: Damn, this demon is tough! He's stronger than any monster I faced! 

Natsu: I don't got time to play around with you! I gotta kick your ass real quick so I can rescue my friends!

Griffon: What?! You were going easy on me?! You dare to take me so lightly?! I am a demon general! Servant of the mighty Mundus! I will not be treated like a merger beast not even worth the effort!

Laxus: Mundus huh? Tell me something. You say you serve Mundus; so why are you helping Vali?

Griffon: You believe I want to aid him?! That insolent fool bares the same wretched blood of the accursed Sparda! However, due to an artifact he stole; Vali has forced me into his service. However, he stated that if I kill you, he will free me from his grasp.

Laxus: So you're a slave.

Griffon: I am NO slave! Once I am freed, I will kill that vile half-demon, and his brother. Then Mundus will greet me with great praise for ridding the blood line of Sparda once and for all!

Natsu: That ain't' gonna happen you overgrown Turkey!

Griffon: You are defending Vali?

Natsu: HELL NO!!! Vali kidnapped my friends! I ain't defending that piece of crap! But you also threatened Drake, and he's my friend! Anyone who threatens my friends will be burnt to a crisp!

Griffon: I've had enough of your tomfoolery! Prepare to die!

He flapped his wings at a rapid rate, creating a large windstorm surrounding him. Griffon then roared as he blasted a massive red lightning bolt straight for Natsu.

Natsu: You first! Fire Dragon Roar!

Natsu unleashed his breath attack, sending an equally large torrent of flame from his mouth. Both attacks collided and were in a large struggle of one trying to overpower the other. 

Both opponents glared at each other as they were still locked in a stand off. But the fire was able to push the lighting back and it blasted straight for the surprised Griffon.

After a large explosion, the smoking Griffon began to fall. But he soon stabilized himself with his wings and was now hovering closer to Natsu. 

Griffon's mind: No. It couldn't be.

He recognized the sensation of that fire; he knew he felt Natsu's attacks felt somewhat familiar.

Griffon: You are no ordinary human. Your attacks, your ferocity. They bear near equal strength to that of a dragon.

Natsu and Laxus looked surprised at Griffon's statement.

Griffon: But not just any dragon. Your fire, that stench you carry, it is the same of that foul beast Igneel!

Natsu's eyes went wide at that.

Natsu: You know Igneel?

Griffon: Yes, that foul creature.

Natsu glared at Griffon for saying that.

Griffon: When Sparda defected to this world, he had found powerful allies; the dragons. When Master Mundus and his forces attacked your world, it wasn't just Sparda who came to their defense, the dragons also aided him in his cause. The Fire Dragon, Igneel was one who stood with and fought with the traitor.

Natsu's mind: Dean's dad knew Igneel?

Griffon: I have clashed with that Dragon on multiple occasions, and yet somehow, I lost each battle. However, I heard rumors that the foul creature was killed by Acnologia. 

Laxus: (surprised) You know Acnologia as well? 

Griffon: In the underworld he was known as the "Dragon Butcher" for his role in nearly causing the dragons' extinction by killing countless dragons four hundred years ago. But six months earlier, the Dragon King was somehow slayed by the hands of humans, or more acutely; you Dragon Slayers. 

Griffon then looks down towards Natsu. 

Griffon: Now thanks to you, I have a new goal that I must achieve before I return to my true master. After I kill both the sons of Sparda, I'll kill you and bring your severed head to Master Mundus! Then, I'll find the corpse of that vile dragon and harvest his power for myself!

Natsu's pupils shrunk with anger and it looked like he was about to pop a vein.

Natsu: I'M NEVER GONNA LET THAT HAPPEN!!! Before you can even think about harvesting power from Igneel, which by the way you can't, you have to kill me first!

Griffon: That is my plan!

Griffon soared and caught Natsu in his claws. He then began to fly straight up towards the night, cloudy sky. He kept flying higher and higher as Natsu struggled in his grip.

Griffon: This will mark the end of you, Dragon Slayer!

Natsu: I don't think so!

His whole body suddenly lit on fire. Griffon was surprised but still held his grip as he continued to fly higher. Natsu's fire began to grow, both in strength and intensity.

Griffon felt the heat begin to scorch his feet until the flames engulfed his lower body, forcing Griffon to let go. He saw the flaming Natsu begin to fall.

Griffon's mind: The fool. He's doomed himself, not even he can survive a fall from this height.

Surprisingly however, Natsu blasted fire from his feet and soared at amazing speeds straight up for Griffon, who charges down toward the Dragon Slayer while coated in red lightning. 

Laxus's mind: This is bad. How are we suppose to fight this thing? I can't his red lightning since it's stronger than mine. (realized) Wait a minute... His lightning color is red. 

Laxus remembered using his Red Lightning Dragon Mode to defeat Wahl during the war with Alvarez. 

Laxus's mind: I could use that form I had six months ago. I haven't used it for a while, but I have to try so I take that feather freak down!

Griffon pushed Natsu down to the ground, as the demon bird then creates huge ball of red lightning.

Griffon: DIE DRAGON SLAYER!!!

Griffon throws the lightning ball at Natsu, however Laxus stand in front of the Salamander to protect him. And after a very hard try, Laxus was able to eat Griffon's lightning. 

Griffon: What?! Impossible! You shouldn't be able to eat my lightning!

Laxus: (smirk) Wanna bet on it? Red Lightning Dragon Mode!

Laxus's body now surges with red lightning which surprised Natsu and Griffon.

Griffon: What the hell?! Are you mimicking my lightning?!

Laxus: Not exactly. It's a power I got by chance during the battle with Alvarez. And it finally became mine!

Laxus dashed towards Griffon and punched him in the face with a fist coated in red lightning. 

Griffon: You bastard! 

Laxus and Griffon clashed with their fist/talon which cause a shockwave of red lightning. 

Laxus: Red Lightning Dragon Roar!

(imagine the lightning is colored red) 

Laxus unleashes a beam of red lightning from his mouth which made Griffon dodged it. 

Griffon: Huh! You missed! 

But turns out, it was all a part of Laxus's plan when Griffon not notice he turn into lightning and go into behind Griffon. 

Griffon: What the- 

Laxus yelled as he thrust a powerful red lightning fist that hit and severed Griffon's right wing; much to the demon's shock. The demon screeched in pain as his left wing began to flap rapidly, to keep him from falling. 

Now it was Natsu's turn who turned into his Fire Dragon King Mode. Natsu stopped Griffon's ascent and sent a massive wave of fire with the sweep of his legs.

The wave hit Griffon's left wing, causing it to explode. Griffon yelled even more in pain and began to scream as he began to fall back towards the castle.

Natsu: I'm doing playing games with you pal! Lightning Flame Dragon Mode!

Natsu had been engulfed in not only fire, but also lightning. As he and Laxus start merging their magic as Natsu's lightning turned into the same color as Laxus's red lightning. They are performing a Unison Raid!

Natsu and Laxus: You're done! RED DRAGON: LIGHTNING FLAME CANNON!!!

Natsu and Laxus focus their magic then sends the attack to Griffon. The attack have the shape of a dragon make from Laxus's red lightning and Natsu's Dragon King flame.

The falling Griffon looked in fear as the dragon soared at him with incredible speeds and pierced through his body.  

Griffon's mind: These boys... the power they wields... they are true... Dragon Slayers.

Griffon's body exploded, in a massive blast of fire and red lightning that rocked and shook the clouds in the sky.

Natsu and Laxus landed on the castle rooftop on both their feet and made a bit of a crater. The flames and the lightning died around Natsu while the lightning shorted out around Laxus as some small flaming pieces of the deceased Griffon fell around them.

Natsu: I hope you saw that Igneel. That was for you. 

Laxus looked exhausted as he fell to his knees.

Natsu: (concern) You okay? 

Laxus: Yeah, I'm spent all my magic power during the Unison Raid. You can go on ahead, I'll catch up once I'm recharged.

Natsu: (nodded) You got it. Now I gotta rescue my friends. (shout) If you can hear me Vali, I hope you saw that too, BECAUSE YOU'RE NEXT!!!

Chapter 18 ended.