They never noticed. Not one of them did.
Not a single one of the lively and bright guild mates of Natsu Dragneel noticed the dark mage. He always made sure his appearance was kept hidden and altered. Of course the matter of his powers unleashing themselves hade never been a problem back then.
He'd been watching him for years, from the day that dammed giant lizard left Natsu. It broke his dark heart to see those olive eyes brimming with tears while he clutched at that scarf. Zeref swore right there and then that he would do anything in his power to bring back that proud demon king. But at least now he was able to get closer.
Throughout the years, it took a good amount of willpower to keep himself from running over to the pinkette, sweeping him off his feet and running away to hide them both. Safe from that brattish raven haired boy who DARED attack his Natsu. Safe from that cruel red haired girl who beat his precious Natsu. Safe from that little white haired girl who walked too close to his Natsu and was too friendly to the fire dem- no... he was a dragon slayer now.
He of course found pleasure in the misfortune of the guild, such as the littlest of the Takeover mage's death. She was too close anyway.
It was a pleasure to watch Natsu grow older and begin to look more and more like the handsome demon he was supposed to be. Of course he couldn't hang around Magnolia constantly as he had other matters to attend; such as misleading those who wanted the black haired mage dead.
Or those who knew of E.N.D.'s existence.
He couldn't risk him.
Then one day, Natsu came back from one of his travels trying to track down the beast that took him from Zeref. But this time was different.
He had brought a blonde haired girl with him.
He didn't stay much longer after that. He could feel his cold veins seep with hate and jealousy at this foolish girl, his teeth bare themselves at each look they gave each other. His powers started spiralling out of control and he could feel his heart sink further and further each day he saw them together.
It wasn't fair...
It wasn't fair!
It wasn't fair that his precious Natsu was taken away from him and his memories, powers and true form, sealed away but this god forsaken girl had to get in his way! It hurt to see Natsu hurt and cry, knowing he was much stronger than any human hoped to realise. It hurt knowing that Natsu was more precious to him than any other of his creations and that he could not even touch him.
He felt himself slide.
He had to stay away after that.
Things around him began to wilt away and die against his will, knew he couldn't go near his Natsu without becoming harmed. He felt his mind slip into depression and he felt no will to live when he knew that Natsu was with that girl.
He eventually found an island he could spend the rest of his immortal days, empty of nearly all life.
He was shocked when the very same guild came to test who were the strongest amongst their candidates after a little over a few months residence. He felt himself fall into despair when Natsu appeared before him, eyes filled with confusion, hostility and without recongisaition.
Maybe he always hoped that Natsu would regain his rightful place beside him if he only could get to see Zeref.
But then again Zeref had done enough to not deserve such luck.
He nearly burst into tears when he thought his powers killed Natsu. At least that dragon was good for something... maybe he'd even thank the scaly kidnapper.
Probably not.
Then the dark guild attempted to capture him and he felt his blood boil. After so long in the darkest crevices of his mind, finally seeing Natsu had put the spark back inside him.
These weak humans believed they could control him? The dark mage, the one who had written a library worth of demons?
They were more like insects. In his rage, he made no move to protect the island when Acnologia came. He cared not if the guild perished, he was certain that Natsu would survive.
After all, he was the strongest creation of Zeref.
He'd do whatever it would take to bring back the flaming demon, king above all others except Zeref himself.
He would bring down that guild that had given him those destructive ideals. It would only hurt more when he regained his memories.
He would kill that girl who he knew would stand in his way in the final battle.
They wouldn't be able to stop him, no matter what Mavis said. He'd massacre them all and bring his precious Natsu back to his proper place, beside him.
No would ever take him away again.
For he was Zeref's and his alone.
His precious Natsu...