My own version of Manga chapters 463-465. Beginning of the war. Where Natsu really ran away on his own (without Happy) to fight Zeref. Gray is worried and so he disobeys Mavis and follows.
SPOILER ALERT! You should've read the chapters before you read this fanfiction.
You're an Idiot... running away, facing Zeref all on your own...
"Gray... Can I ask you to place your trust in Natsu?" Mavis asked.
"It's not like I don't believe in him. Him diving in there alone... I'm just a bit worried about him..." he said scratching his head.
I need to follow him...He's in danger... I can feel it...
He heard Juvia saying something and then Happy shouting "Aye Sir!" but he really didn't care. He needed to get out of here. Mumbling something about being right back he left the guild hall walking slowly. The plan of the First could be the best way to win this war, but leaving Natsu alone to face Zeref and maybe let him die was not an option for him. Natsu meant more to him than anything.
As soon as he had left the guild hall and thought nobody was paying attention to him anymore, Gray sped up to the fastest running speed he could muster.
When he reached the battlefield, Natsu and Zeref were already facing each other. Cold fear crept up his spine. Natsu had taken off the bandage of his right arm. Gray could see a dragon tattoo adorning his muscular arm.
For now he decided to hide and just observe. Maybe Natsu really had an ace up his sleeve to defeat Zeref. He lay down within earshot on a nearby rock.
"Blaze Dragon King Mode!" Natsu shouted.
Suddenly flames lit up everywhere; Gray was only barely able to encompass himself in an Ice Make Cage to avoid taking too much damage. His ice magic was not able to withstand those burning blazes for long. Just when Gray's ice had melted away completely and he thought the fire would burn him to a crisp, the fire dispersed and he could see clearly again.
Natsu's dragon tattoo was surrounded by steam; he was panting heavily but took a fighting stance again. Gray couldn't help but admire his courage and determination.
When the mist around him evaporated, he could see Zeref standing weakly. It looked like Natsu really had landed a critical hit.
"Lend me your power... one last time, Igneel... one final blow is all I need..."
Zeref straightened himself.
"You truly have attained marvelous strength, Natsu... I always believed that you might be able to defeat me one day. However, you were a bit too late..."
Finally facing the power that could possibly kill him, Zeref decided it was the time to reveal the whole truth about himself and Natsu and the Dragon Slayers. He held a long monologue in which he unveiled Natsu's history and that of the four other Dragon Slayers chosen for the sole purpose of defeating Acnologia: Gajeel, Wendy, Sting and Rogue. He explained that the dragons had purposely concealed themselves in the bodies of their respective Dragon Slayers and traveled to the future by using Eclipse. He also revealed that Natsu was his little brother who had died and been revived by him as the Demon E.N.D., short form of Etherious Natsu Dragneel.
Gray's head spun, overwhelmed with all this new information and the question if he could believe them or not. At one point Zeref had pierced the book of E.N.D. with his magic and Natsu had shouted in pain as if he himself had been pierced. There had to be some truth to his story. It all sounded so perfectly logical. But that meant that Natsu was a Demon, and not just any Demon, but the strongest one. Silver, his father, had given him the power to slay Demons and had asked him to destroy E.N.D. He had also mentioned the type of magic E.N.D. would wield: it was fire.
I'm sorry father, but I can't fulfill your wish...
"You are a Demon of the Book of Zeref. Once you kill me... You will cease to exist as well."
"LIKE I GIVE A DAMN!" Natsu shouted, racing towards Zeref with fiery resolve.
This was it. Gray was done waiting and listening. Natsu was about to kill Zeref and himself. Now was the time to act.
He formed an Ice Make Fist and rammed it into Natsu's side so hard that it sent him flying for several hundred meters. He raced over to him, grabbed him at his collar and hit him hard in the face. Natsu was so stunned that he couldn't react or defend himself.
"You asshole! What do you think you were doing just now?! Sacrificing yourself without thinking about your friends?! I'll not let you do that, I'm taking you with me now, you hear me? I'm not gonna let you die!"
Natsu's expression had slowly changed from stunned to wretched. When Gray picked him up and put him on his back, forcing him in the piggy-back position, he didn't complain. Gray ran as fast as his feet could carry the both of them, not caring where he was heading, just far away from Zeref.
When he reached a peaceful spot in a forest near a small lake, he finally stopped and dropped Natsu onto the soft grass. He dropped to his feet next to him, grabbed his collar again, leaning over his friend.
Somewhere on the way, Gray had started to cry. He noticed it only now when his tears fell on Natsu's shirt, soaking it.
"I promised Juvia I would give her an answer after all this ended." He wanted to look in Natsu's eyes, but they were hidden by long and messy strands. Tears were constantly streaming down Gray's face. He didn't even try to stop them. He didn't mind right now.
"This is my answer... My heart belongs to somebody else. It was never hers and will never be." His tears were falling at rapid speed now; some of them hit Natsu's face as Gray was trembling uncontrollably. He let out a heartbreaking sob. He couldn't control his emotions any longer.
"Don't you ever do something like that again! We don't want to lose you... I don't want to lose you! I- I couldn't take it... My heart... couldn't take it..."
His body was shaken by heavy sobs. His grip on Natsu's collar had weakened to the point where his hands were merely resting against Natsu's collarbones.
"Gray..." Natsu's hand reached up to cling to Gray's face and wiped away the tears with his thumb. The ice mage leaned into that touch, snuggling comfortably against the rough hand of the fire Dragon Slayer. His tears didn't subside though, if anything he cried even more than before.
"Can you promise something to me?" Natsu asked sternly, his eyes still hidden by his hair.
"Promise what?" Gray wanted to know between sobs.
Natsu lifted his upper body, tilted his head and before Gray realized what was happening their lips were softly touching each other. It was a chaste kiss, their lips wet and salty from the tears Gray was shedding. Natsu drew back before Gray had the chance to react, licking the salty tears from his lips. He sat up next to the ice mage, turning his head and leaning his forehead against Gray's, the tips of their noses slightly touching each other.
"After this is over... promise you won't take back those words or what they mean. Promise you won't back down and run away from me."
Gray's heart was racing, he felt dizzy. Their gazes were locked on each other, Natsu's eyes full of emotions and begging him to give a positive answer.
"I... I promise..."
Natsu smiled fondly, gently pushed him back on the grass and placed himself on top of Gray. He slowly lowered his face. They didn't break eye contact until their faces were so close to each other that their breaths mingled. When Natsu closed the distance between them, their eyes fluttered shut.
Their lips touched only barely at first, Natsu wanted to savor the tingling feeling of anticipation. Gray let him have his way for some seconds but he just couldn't wait any longer, so he leaned up and sealed their lips softly together. Natsu sighed with pleasure and let his body sink completely on Gray's. He caressed the cheeks of the ice mage tenderly while Gray embraced Natsu tightly, pulling him in even more and intensifying their kiss. He never wanted to let go again.
Gray wanted to feel more, so he parted his lips and nibbled softly at Natsu's bottom lip. The fire mage couldn't hold back a moan of pleasure.
After what felt like only seconds to them but in reality had been several minutes Natsu broke the kiss. Gray followed his lips trance-like, but when he noticed that Natsu was sitting upright again and their special moment had ended, he pouted.
The Dragon Slayer jumped to his feet and held out his hand for Gray.
"Let's continue this after we've won the war. C'mon, there are some Alvarez' asses waiting to be kicked!"
Gray couldn't help but smile. He accepted the offered hand and was pulled to his feet. Everything that separated him from his future with Natsu was the empire of Alvarez. He felt incredible power flow through his body. They could do this!
"I'm all fired up!" Natsu exclaimed as they started dashing towards the battlefield.
On that day a powerful weapon was created. A weapon with the power to take down an emperor and his empire, to end a war and spread peace all over the world. This was the power of two Fairy Hearts beating in unison.