Two and a half months earlier...
Arthur, half of the way down to the training field, slowed to a stop. "Oh, come on," he said to himself, poking at his empty scabbard. Merlin must have forgotten to put the sword back in its place after cleaning it again. Rolling his eyes, he turned around and went back towards the way he came.
Arthur pushed open the heavy door, and stopped at the sight of Merlin cleaning his chambers. Or, rather, he was stopped by the sight of Merlin flicking his hands around while Arthur's chambers cleaned themselves. His back was to Arthur all of the while, and as a result he didn't see the prince standing there. Watching Merlin. Using magic. Merlin was using magic.
And, all the while he was doing this, Merlin was humming. Humming. Humming some nonsensical, tuneless, Merlin-like melody as he sent boots running off against their will and dust crawling out the window. He stopped, surveying the now clean room. Arthur's hand went for his sword, just to remember the very reason that he'd come back here was to get his sword. Merlin started to turn towards the door, but Arthur moved away, pressing himself into hiding out of sight.
Merlin poked his head out the door, looking around warily. Arthur didn't even dare breathe as Merlin vanished back into the room for a moment. Then, whistling beneath his breath, his servant bounced out of the room and down the hall, still humming that odd tuneless song.
Arthur left his alcove and, as his first action, proceeded to tear apart his room searching for any signs of magical threats. It wasn't long before his room was trashed, belongings spread out all over the floor and a pair of pants dangling off of the curtain rod. Arthur stood in the middle of the disaster, staring blankly into space. Some sort of dangerous magical creature had, obviously, kidnapped Merlin and proceeded to take his place in the castle with no doubt nefarious intentions. At least, Arthur hoped the idiot had only been kidnapped. The alternative was...unsettling. Scratching his head, he considered his options.
Any other time, Arthur would have gone straight to Gaius, but Gaius wasn't here, not for another week, at least. And if he went to his father, then Uther would just have Clearly-Not-Merlin executed, leaving Arthur with no way to track down Real Merlin.
Nodding to himself, Arthur walked off in the direction Clearly-Not-Merlin had set obliviously off in, completely disregarding training. Before he went to the knights, though, he wanted proof and a solid argument.
It was two days later, and they were to leave on patrol the next day. Arthur sank down on the side of his bed. Two days, he had spent the last two days following Merlin everywhere the servant went. He had assigned him ridiculous, impossible tasks, and with grumbling Merlin would complete them, each time using magic. He'd also spoken to Gwaine and Lancelot and the others, but none of them had noticed anything out of the usual about Merlin. And the more Arthur thought about it, the more he came to suspect that Merlin hadn't been replaced by a dangerous magical creature, but was actually a dangerous magical creature himself.
A sorcerer.
Arthur buried his face in his hands. The last time he'd been this stupidly terrified was when he thought that Guinevere was going to be burned on accusations of witchcraft. Like Merlin would be if he were caught.
Arthur didn't think for a second that Merlin would try and hurt Camelot. Maybe he'd come to the city with such intentions initially, but Arthur couldn't be bothered to care at this point. He refused to believe that the Merlin he knew, the only one who had dared to just tell him to his face what he thought, could do anything to remotely attack Arthur. If he was going to, surely it would have happened by now.
But that didn't change the fact that Merlin was a sorcerer. In Camelot. He was actively using and practicing magic within not only the kingdom, not only the city, but right inside the castle walls. Merlin was not safe. Merlin, smiling and stupid, lying and deceiving, was not safe here.
That night, Arthur didn't sleep. There were too many visions of pyres being lit dancing through his head, phantom screams rising up from the courtyard. He gave up any pretenses of sleeping when the sun rose. He pulled open the curtains and stared outside. The courtyard was empty, but it was way too simple to imagine the executioner's block, the raised axe, Merlin on his knees, eyes shut as the blade came crashing down on his neck...
Arthur pressed his forehead against the glass. There was only one thing that he could do.
Merlin came waltzing in, tripping slightly as he crossed the threshold as Arthur was struggling to put his shirt on. "Oh," said Merlin. "You're up early."
Arthur shut his eyes for a moment, and braced himself for what he had to do.
"Here," said Merlin, reaching out towards him, "Let me –"
"Don't touch me," he snapped, brushing Merlin's hand away. He didn't miss the look of hurt that crossed Merlin's – not Merlin, couldn't think of him as clumsy Merlin, the sorcerer's – face. He had to do it now. Now. Before he lost his nerve and condemned Merlin to an inevitable execution at the hands of Arthur's father.
Arthur didn't even know what he said, then. What words he threw like daggers at Merlin, each one sharp and aimed to hurt. Merlin looked a little bit like he was dying. Arthur wondered how many times Merlin had thought about this moment, where Arthur found out. How many different scenarios the idiot had come up with, and of those, how many of them resembled the one that Arthur was forcing him through right now? So, Arthur paid little heed to the venom he spat out, and guessed that Merlin would remember each and every word for the rest of his life. He even gave Merlin a shove, at one point.
At the end, when Merlin's eyes were filled with barely contained tears, Arthur turned away and said, "Get out of my sight, Sorcerer. And if I ever see you again, I will kill you."
Merlin didn't move. Just said, in a voice that was broken, "Arthur –"
Arthur squeezed his eyes shut and bellowed, "Go!"
Merlin, each breath hitching, scrambled to his feet and ran for the door. Arthur could hear his footsteps swiftly crossing the floor. Then the door slammed.
Arthur crossed his room to the chamber pot, and threw up into it. "It's for the best," he told his empty, silent room when he'd finished, "It's for the best."
But for who?
It wasn't until the end of the day – after training and meetings and not thinking about Merlin's footsteps leaving the room – that Lancelot tracked him down, face grim and jaw set. "Something's wrong with Merlin. I saw him earlier but he refused to tell me," the knight said, walking into Arthur's room, not bothering with any preamble. The prince didn't answer, just kept gazing out the window. "And now no one can find him anywhere. Have you seen him?"
"Did you know?" he asked, voice rough.
"Did I know what?"
"About Merlin's magic," said Arthur, twitching his gaze over at Lancelot. "Did you know?"
Lancelot didn't answer, but the thunderstruck expression that flew onto his face was answer enough. Arthur could also see the pieces flying together in the knight's mind. "What did you do?" Lancelot asked, "Arthur. What have you done? Where's Merlin?"
"He's gone," said the prince, "And – And he won't be coming back to Camelot."
"What did you do?" said Lancelot again.
Arthur turned, and as he swept out said, "You should have told me."
Lancelot wouldn't say another word to him save for clipped answers to direct questions for the next two months. But he was talking to Gwen, because the very next day she came up to Arthur, and said, "What's happened to Merlin?"
"He's left."
"Why?" said Gwen, "Where did he go?"
"I don't know," Arthur said, choosing to answer only her second question, "Ealdor, maybe. I don't know."
"How can you not know?" she said. "You're his best friend."
"No," he said, "No, I'm not. Not anymore."
"Arthur," she said, staring at him in shock, "How can you even say that? Lancelot mentioned that –"
"Merlin practices magic, Gwen," said Arthur, and her words faded off as her mouth dropped open.
"No," she said, "There must have been some mistake, he...someone's framed him, then. Merlin isn't a sorcerer."
"It isn't a mistake," said Arthur, "Ask Lancelot if you don't believe me, he was in on the whole thing."
"But he's Merlin," said Gwen, tears welling in her eyes, "He can't be a sorcerer."
"I saw him at it," said Arthur.
When he didn't continue, she said, "You haven't...I mean, Arthur, you didn't turn him in, did you?"
"Of course I didn't," Arthur said, "But he won't be coming back."
"What did you do?" Gwen whispered.
"I made sure he wouldn't die here," said Arthur, pushing past her, not looking up to her face where he knew she was crying, "I made sure he wasn't coming back."
A week passed, and there was no sign of Merlin. Gaius returned to the city, and Arthur remained in his chambers, watching from the window. The old man was barely off his cart and Lancelot (with Gwaine just behind him) was crossing the courtyard. Arthur watched the discussion the three had, could see the seriousness in their posture. Gwaine had known, as well? Or had he found out from Guinevere? Arthur wouldn't have been surprised in either case.
Gaius looked up towards his window. Arthur stepped away, the guilt threatening to choke him. "It was for the best," he said again. This time, he was less convinced.
"Sire," said Gaius, approaching Arthur after a council meeting and not bothering to pretend, "At least tell me if he lives or not."
"He lives," said Arthur, after a moment. "But...But he wouldn't have lived had he stayed here, Gaius. I didn't have a choice."
"Yes, you did, Sire," said the old physician, "I only hope that you made the right one."