The Steady Creep of Darkness
by Contrail
Disclaimer: Fairy Tail belongs to Hiro Mashima, and is being used without permission. No profit is being made from this story.
The first signs that the curse power Gray had absorbed from Mard Geer's curse was tainting him weren't the blackened patches of skin spreading across his right arm. No, they were more subtle (insidious) than that. It started with him getting a little frustrated over his progress towards defeating END. When the guild had disbanded, he'd decided that he needed to work on getting stronger before he tried to go after END. (After all, they'd had so much trouble with Tartarus as it was, and their master, Zeref's strongest demon, was surely much stronger than them.) But even though his power was growing steadily with the training he was undergoing, it felt like he wasn't improving fast enough.
Gray started to doubt whether the path he'd first chosen was the right one. The knowledge that somewhere out there the Book of END existed was waiting for him to find it was like a growing itch at the back of his mind. He didn't have time to dawdle - who knew when Zeref would finally decide to unleash END on an unsuspecting world? He tried his best to soothe that itch, telling himself that it would be useless to search without any knowledge of where it might be to guide him, and futile to find END if he hadn't the strength to destroy it. But the feeling that he should be doing something more about END always crept back upon him, stronger each time.
Juvia noticed that he seemed more preoccupied than usual and started questioning him about what might be bothering him. Most of the time he tried to reassure her with a smile, telling her that it wasn't anything important. A few times he asked whether she thought there was anything more they could be doing to get closer to the goal of destroying END. She always did her best to assure him with sensible arguments that they were doing all they reasonably could at the moment. She couldn't sway the restless urgency to complete the mission his father gave him, to avenge everything he and his family had suffered through growing within him, though.
When he noticed the jagged strips of black flesh appear on his arm, he looked at the discolored patches with a kind of dull surprise. Were they side effects of his Devil Slayer magic's increasing strength…? After a moment he decided it didn't matter as long as it didn't hinder him in his efforts against END. He needed to accomplish that, regardless of what happened to his body. In response to Juvia's concern about the change, he gave her a confident smile and told her not to worry, but he couldn't quite meet her eyes as he did so, knowing that her worries were probably justified. He just couldn't bring himself to care that much about what happened to himself anymore.
The change did give him an idea, though. They'd considered trying to get information about Zeref's whereabouts by defeating and questioning dark guilds before, but had dismissed the idea as too dangerous. Any guild close enough to Zeref to have any useful information would be too strong for them to feel confident about being able to defeat them with just the two of them. But if he could convince one he'd gone dark and join them, then maybe… It'd be a risky path, still. He'd likely have to do things to prove himself that would taint him. But it would bring him closer to his goal, finally.
What about Juvia, though? If he told her about his plan, he knew she would refuse to let him go alone. Her presence would make things riskier for them both, though - whatever group he tried to get into might believe one Fairy Tail mage going rogue, but two? He'd be distracted by worrying about her. And he wouldn't let her ruin herself helping him. So he couldn't tell her what he was going to do if he did this. He felt a pang in his heart when he thought about how hurt and angry she'd be when she realized he'd left her all alone. But he had to follow any path that'd lead him to END.
Gray started leaving their home by himself, nosing around for any leads he could find about powerful dark guilds. Apparently after the fall of the last of the Balam Alliance, a new group was taking advantage of the power vacuum to make their move. The group, named Avatar, was apparently more of a cult of Zeref worshippers than a dark guild (though he honestly thought that there was little practical difference - Grimoire Heart and Tartarus had pretty much worshipped Zeref as it was).
As he chased different leads on them, he was gone more and more. Once he was sure he had one that would lead him to them, he returned home one last time. After Juvia went to bed, he waited for her to fall asleep, then went into her room to see her one last time. As he watched her sleep, pale blue hair spilling across her pillow in waves, a happy little smile gracing her lips, for a moment he wondered what he was thinking, wanting to leave her and the life they'd built for themselves here. It was a peaceful one full of contentment, even with the shadow of his eventual confrontation with END over their future.
But then the anger within him flared once more. How could he be at peace while END still existed? When it had not been made to pay for what had become of his father? While it was still a threat to everything he cared about? So he turned his back on Juvia and closed the door behind him. Making his way to his own room, he made his way to his bedroom to pack up some clothes and other necessities. Everything else he wouldn't need where he was going. Maybe he'd make it back someday to use them once more… or maybe not. It didn't really matter, not in the face of what he had to do.
He fingered the somewhat distinctive pendant he wore around his neck. Gray remembered he'd been drawn to look at it amongst the jewelry in a stall he passed by because it reminded him of the crosses his father used to wear. The merchant had told him that it was a replica of a legendary stone, one said to have been created to purge darkness from the world. With a snort, he took the necklace off and placed it in the drawer of his nightstand. Even if it was only a fake, such a thing didn't belong on the dark paths he planned to walk. Maybe Juvia'd find it and it would lend her its light.
Closing the drawer, he made his way out of the room, then out of the house and into the darkness, his heart determined to fulfill his promise to his father no matter the cost to himself… or anyone else. (And he never considered if this is really what his father would have wanted for him.)
I know that what Gray was thinking when he left is a popular topic now, but I thought my perspective on it was different enough to be worth putting out there, since chapter 426 has finally shown enough of him for me to get a decent read on what I think is going on. I really don't think he's entirely not himself - taking his statements to the rest of Avatar at face value, his goal (defeating END) is still what it was, he's just taken it to the point of obsession. Which is probably from the curse power from absorbing Memento Mori amplifying his existing darker emotions (particularly an understandable anger and desire for revenge in regards to END). And I think the change was slow and subtle enough that he never even realized that his mind was being affected as well as his body. :^/