I'm... so... sorry. Beginning of the fic, Natsu and Gray are little tikes.
Disclaimer: I do not own Fairy Tail, so you're all lucky.
Dark eyes stared into the clear edge of the river, watching as tadpoles zipped about near the bank. Their movements were swift and seemingly without meaning as some circled back to where they had just been and others just kept swimming toward the end of the water. He wasn't much concerned with them, he was far too deep in his thoughts. He hadn't shown up to the guild that day on account of being too busy thinking about his father's old lectures.
"Remember, Natsu." Igneel once told him, his booming voice drowning out all other sound. It was no wonder everything he said really stuck in the young boy's mind. He was impossible to ignore. "Plenty of bad people were once good. You may find yourself on a path of darkness someday if you are not careful about where you step."
He pondered that concept, his adolescent mind turning it over this way and that. How could someone who was evil be good at one point? Weren't the bad guys bad and the good guys good? It was starting to hurt his brain. Why would Igneel be so concerned with good and evil anyhow?
"Oi, Flame-brain. Whatcha' doin' out here by yourself?" Natsu was pulled from his thoughts, glancing over his shoulder to see his mutually proclaimed rival yelling at him from a few yards away. The half-nakedness of the boy was hardly worth noticing.
"None of your damn business!" Natsu barked back, on his feet in mere seconds. Though his previous thoughts weren't lost on him. He didn't give Gray much time to respond before he abruptly implored, "Gray, can I ask you a question?"
Gray's expression quickly turned from challenging to surprised, one fine brow raising as he walked over to meet Natsu. "First you skip the guild and now you're asking for advice? What are you, sick?" He joked halfheartedly, attempting to lighten the dark mood.
"I'm serious, Gray." Natsu replied, his eyes a promise to prove him sincerity. Gray averted his gaze as he took a seat in the grass, Natsu soon following suit. Despite popular belief, Gray and Natsu had their moments of peace. Even though they were constantly bickering, they were each others' closest friends. They spent the majority of their time together, after all.
Natsu's gaze returned to the river. "If I ever turn evil, will you be the one to kill me?" Natsu asked in complete seriousness, sounding rather contemplative.
"Eh? Where's this coming from?" Gray asked, caught entirely off guard by the question. "What're you on about?"
The rose haired boy looked up at him, calm. "It's something Igneel taught me. He said all bad people were good once. If I go bad, will you stop me?"
"You dumb bastard." He was frowning, which made Natsu's brows dip in anger. Gray then began to grin, showing off his teeth. "No one else would be able to." He jabbed before assuring, "Yeah, I'll take you out if you go bad. Only if you promise to do the same for me."
Natsu smirked in response, seemingly satisfied with that response. "It's a promise."
Trembling. He felt his hands trembling. His fists were balled tightly at his sides, his nails biting deep into his palms and painting his knuckles white, and they were shaking. He stared, all but petrified, in a mix of outrage and surprise at the figure standing before him. Eyes wide, drinking it all in. Every last bit.
His back was turned, but the rosy hair was unmistakable, even with those demonic horns jutting from either side of his head and ending angled straight up with a sharp point. His clothes were tattered and torn from earlier battles, a stark contrast to his new clipped demeanor. At his feet lay an open book, one emitting magic power in a coiling wisp of gold that his body endlessly soaked up. The increase in the man's own magic power had been rather appalling, the sheer strength of it enough to send nearby mages to their knees. It was obscene.
"You dumb bastard." Gray muttered, allowing his bangs to fall over his eyes and cast a shadow. He felt tears brimming, uncaring of whether they decided to fall or not.
The man whom he had spent the better portion of his life growing up with; arguing, taking jobs, growing stronger, and becoming friends with, turned to stare at him. The sadistic grin on his face was unlike anything Gray had ever seen. It didn't feel right, watching this thing possess the body of his nakama, someone who knew better than he knew himself. To parade around in his friend's... his best friend's body like that, it made Gray sick.
"All bad people were good once. If I go bad, will you stop me?"
"I will kill you, END." Gray promised, discarding his shirt and revealing the markings that consumed most of his arm and a portion of his torso. It spread up his neck and across his face, making him look particularly ominous. "I'm the only one who can."
Sorry, I just had an idea. I don't really have time to write this into a full-blown fic but anyone is welcome to run with the idea. I made myself sad.