Wow, fuck.
I never imagined this would have such a long gap between updates.
Hopefully you can forgive me. I don't know when the next update will be, but hey, I hope I won't leave it quite this long.
Here's Arthur's portion of things. Mordred next, of course.

"Sire?"

Arthur looks up, eyes meeting Leon's across the room. "Come in," Arthur says, setting down his pen and gesturing for him to come forward.

Leon seems wary, nervous. Arthur feels concern and nerves of his own running down his spine. "What's the matter?" he questions.

"Sire, I don't mean to suggest - that is, I don't want to - Sire-"

"Leon," Arthur leans forward, setting his hands on Leon's. They're ice cold but Arthur ignores it. "Whatever you have to say, I'm sure it'll be alright."

"How do you know that you can trust Merlin and Mordred to not betray you?" Leon says in a rush, deflating as soon as the words are in the air.

Arthur's stunned, for his part. He had never considered that Merlin, in particular, would betray him. Never. Not even once.

But Leon, who doesn't know either of them as well as Arthur, is bound to be more suspicious. Arthur shakes his head, leaning back. "They won't," he says, hoping his voice comes out strong.

"But, sire, you don't know that," Leon presses, and Arthur holds his hand up, silencing him.

"I do know that," Arthur says. "Merlin has never given me reason to doubt."

Leon's lips are thin, his face stone. Arthur softens. "Trust in them. Moreover, trust in me."

At this, the knight deflates. "Of course, sire. Absolutely."

…

Leon's words haunt him that night, no matter how much he wishes that they didn't.

"You're brooding," Guinevere accuses, rolling over to meet him in the bed. She nuzzles close, her nose along his jaw line. "Talk to me."

"I just need to trust them," he says. Guinevere, bless her, knows exactly what he's talking about.

"It'll be okay. Arthur, they have remained loyal to you. Mordred came back to your side when you banished him, and Merlin has stayed for over a decade despite having reason to leave. Why would they do so now?"

Arthur closes his eyes, taking a deep breath. "Yes, of course," he breathes. "I just - Leon. He brought up a point. How do I know they want to stay? Did you see their faces when I drove them away, even though we all knew it was a ruse? How do I know they will return friendly?"

"It's like you said," she murmurs, "you need to trust them. When have they actually betrayed you?"

"You're right, of course you're right."

"I'm your wife. I'm always right."

Arthur snorts and Guinevere lets out a little laugh.

…

"We have tracked the bandits to a cavern not far from where the druids live, sire," Gwaine says, rocking on his feet.

Arthur nods, gesturing for him to continue. Gwaine takes a deep breath. "The good news, sire, is that they've all been killed." Arthur frowns, leaning forward. "The bad news is that whatever killed them is clearly more than we can handle. Whatever was there has never bothered us before, is seemingly docile, but I feel we must keep an eye out anyway."

He nods. "Yes. Thank you, Gwaine," he says, waving him away. Gwaine gives him a dopey smile with a mocking bow before turning on his heel. Arthur shakes his head in amusement, returning his sight to the paperwork regarding the cows.

"Arthur."

He jumps, head shooting up, quill clattering to the table. "What the -"

"We've got her," Merlin says, grinning. "She's let us in. I thought you might want to know, but I've not got much time. I'll give you more information later."

The mirage dissipates, and Arthur can't drag his eyes away from where the see-through image of Merlin had stood, looking a little worse for the ware but definitely present.

…

"Can warlocks use magic to move from one place to another but be sort of invisible?"

Gaius turns around with a befuddled look on his face. "I'm sorry?"

Arthur makes a noise of frustration and sits heavily on the bench next to the table. He waves his hands around, searching for words, but these movements make no progress in his attempt to make Gaius understand. "Merlin appeared before me in my office but he was see-through," he says, defeated.

Thankfully, Gaius makes a sound of comprehension. "You're speaking of projection, sire," he says, turning back around to his books. "Merlin transported his consciousness into your office and spoke to you using telepathy in a way you would understand."

Arthur stares at him blankly. "I don't understand," he states, dropping his hands. Gaius huffs in amusement.

"Did you receive his message, sire?" he asks.

"Well, yes, but-"

"Then you understand all you need to," Gaius interrupts, turning around to face him again. His face is kind, if still exasperated. "There is only so much I can explain to you, since you do not practice magic yourself, any further attempts would only confuse you more. To put it more simply, Merlin thought about you, and made his image appear before you."

Arthur is certain he's just been insulted. He's not sure whether it was intentional or not, or whether the physician knows but wants to be sure that Arthur himself does not. Either way, he decides to let it slide, too worried about Merlin and Mordred to really care.

He stays quiet, just nodding in response to Gaius' explanation. The old man watches him for another moment, then turns around, apparently deciding that Arthur is not going to say anything more.

And for a while, he doesn't. He watches, his mind a world away, and when he speaks, he does it almost without knowing. "Merlin and Mordred will be okay, right? They'll return just the same?"

Gaius stops. "What is this about, sire?" he questions, facing Arthur once more.

He shakes his head helplessly. "I… I have been betrayed so many times. What if Morgana gets to them? What if I am the one being played?"

Gaius doesn't seem offended on Merlin's behalf, which is a reaction that Arthur half-expected. "Has Merlin explained to you your destiny?" he asks, which seems pretty random to Arthur.

He answers anyway. "I am to rule Camelot," he says, then furrows his eyebrows when Gaius shakes his head.

"That is not all," he says. "Your destiny is to build Albion alongside Emrys. You are the Once and Future King of Albion, Arthur. And it is Merlin's responsibility to get you there. He believes in you and your destiny too much to ever abandon it of his own free will."

Arthur is getting tired of surprises. He stares at Gaius, shocked. Surely he is joking.

But the man doesn't look like he's jesting. He looks solemn and patient. "…all of Albion?" Arthur manages, and Gaius nods.

"Merlin reacted much the same way you did, though he was a bit more dubious of your part," Gaius says, which is probably meant to be reassuring. Arthur's nodding before the second part of the sentence sinks in. He jerks.

"Hey! What?"

Gaius smirks lightly. "He'd only just met you," he says.

Arthur deflates, nodding with a short laugh. "Yes, okay," he agrees.