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Arthur's still nestled comfortably in the clutches of sleep when Merlin comes bursting into his room yelling at him to wake up, so it takes him a moment of disorientated flailing to find his pillow and chuck it at the vague direction of the noise. By the time he opens his eyes, Merlin has already crossed the room, having easily ducked underneath the projectile, and is currently attempting to yank the covers off him.
It takes a moment for Arthur to realise that Merlin is laughing.
Normally, he wouldn't have given the fact a second thought. Back in Camelot, back in a different, ancient life, he would have been worried if Merlin hadn't been laughing. It was just the way he was - his stupidly cheerful, clumsy, warlock manservant. But from the second he had been returned to the world of the living, woken up after his death, fifteen hundred years into the future and faced with the same face he still remembered from his dying moments, he had known, instantly, that the man standing frozen in front of him wasn't the Merlin he knew. The look in his eyes, a carefully detached, nearly hollow expression that he could only imagine had resulted from the centuries of conflict and loss and burdening knowledge, almost made Merlin look like a stranger.
Then Merlin had whispered his name like a long-awaited answer to prayer, those empty, desperate eyes filling with tears, and Arthur had thought, It's him, it's definitely him. Even with the unimaginable kind of anguish that he seemed to emit that never quite went away even as Merlin brought Arthur to meet his knights, and Guinevere, and even Morgana (the weeks following her tearful apology were about as awkward and strained as it could be, but it was worth it to have her back the way she used to be before her fear of Uther and Morgause's influence turned her), Merlin was still Merlin.
But Arthur never saw him really, truly smile the way he did before.
Until now.
He stares, bewildered, barely registering Merlin's words. He looks thrilled, elated, in a way that he hadn't seen often even back in Camelot.
Something really good must have happened, he thinks belatedly. Did someone else come back? Was the world-threatening crisis he returned for somehow already averted?
Naturally, of everything possible he thought of, he didn't expect the words that next came out of Merlin's mouth.
"Arthur! Arthur, I'm going to die!"
Arthur doesn't think he's ever gotten out of bed so quickly before. The last remnants of sleep evaporated from his mind, he grabs Merlin's wrist, spins him round to face him, and yells back, "What??"
Merlin falters, his smile frozen on his face.
"Ah," he says. "I see how that must have sounded. My bad."
"You're what???"
Merlin laughs again, gently shoving Arthur back onto the bed. "Calm down," he says, still looking flushed with happiness. "I'm not going to die now, obviously -"
"You're going to die??" Arthur doesn't let go of Merlin's wrist, suddenly overcome with the bizarre notion that if he let go, Merlin would turn into dust and float away. That doesn't make sense, he tells himself sternly, but only loosens his grip slightly.
"For god's sake, Arthur, let me explain -" Merlin huffs, and the expression of fond annoyance on his face is so painfully familiar, almost as if they're back in Arthur's old chambers and they're arguing over the apparently "ridiculous" amount of chores that Arthur sets him.
"Explain," Arthur demands. Merlin rolls his eyes.
"Okay, I'll make it simple." He grins broadly. "I'm not immortal anymore!"
There's several, prolonged moments of silence, in which Arthur squints at Merlin and wonders if he hit his head.
"Uh-huh," he says slowly.
Merlin groans, flopping down on the bed besides him. "My god. Don't you get it?" He points at himself. "I'm ageing, Arthur. Like you, like everyone else, like how humans normally age! And, if all goes well and nothing bad happens, I'll die at the end! For real, this time!"
He looks positively ecstatic. Arthur watches him in concern.
"If all goes well ... you die?" That doesn't sound like something going well.
Merlin frowns at him. "Goodness. I forgot how you mortals always were with death."
Arthur's pretty sure he looks very offended right now.
"Us mortals -?"
Arthur's cut off by Merlin shushing him, a little of his joyful exterior cracking. "Arthur," he says quietly. "I've been watching everyone around me, everyone I let myself love, grow old and die for over a thousand years while I stayed the same. I've had to bear all these memories, hundreds of lifetimes worth of - of pain, all the while remembering how you and everyone I knew from Camelot died around me." He laughs again, but this time it's muted, a little broken. "I thought I would have to remain here, until the last human died and I was left alone on this empty planet."
Once again, Arthur is reminded of the staggeringly different lives they've led.
"Merlin," he says faintly. What do you even say to something like that?
I'm so unequipped for this, he thinks. Lancelot was usually the one who was alive at the time to deal with the bouts of temporary insanity that Merlin allegedly suffered from every few centuries. Side effect of living for over a thousand years, he'd said.
"That probably sounds weird," Merlin sighs. "Communication - you'd think I would have gotten better at it after fifteen hundred years of being on a steadily overpopulated planet."
"No - Merlin," Arthur fumbles for a second, tugging Merlin's wrist slightly to get him to face him again. "It's not - it's not weird. I can't even imagine - nobody could, the things you went through, alone. And now you have the chance to not have to go through another lifetime of pain. Of course you'd be happy. And if - if you're happy, then I'm happy." He gives him a wry smile, which Merlin returns tentatively. "But don't get the wrong idea," he continues. "If you think this means you're allowed to throw yourself in front of every sword the next time we're out on a battlefield, you're sorely mistaken."
"Guns, Arthur, people use guns now -"
Merlin is laughing again, and Arthur sits back, relieved.
"You're WHAT?!" Gwaine shrieks, and both Arthur and Merlin wince.
Gwen seems close to fainting, and Lancelot has turned deathly pale. Arthur turns to Merlin, and smacks him round the head.
"Why did you word it that way again? You already saw how I reacted!"
"Oops," Merlin says unhelpfully, grinning. "Sorry, guys. What I meant to say is, I'm no longer immortal. I'll age - I'm already ageing, slowly. And - like I told Arthur - if all goes well, I'll die at the end, like how people normally do!"
Gwaine collapses into a chair and continues staring at him, jaw slack. Leon reaches over and pushes his mouth shut.
Already looking faintly amused, Merlin huffs. "Come on, guys. What's the big deal? You've all died, like, ten times already."
"That's not fair," Percival mumbles.
"This doesn't really change much." Merlin smiles reassuringly at Gwen, who is still covering her mouth. "I still have my magic, and it's in fine working order. Since Arthur returned, England is apparently in it's "greatest time of need" or whatever, and we still haven't figured out what the threat is, yet -"
"COVID-19," Elyan offers.
Merlin's expression changes to one of dawning realisation. "Yeah, Arthur, you should become a doctor and find the cure, maybe that's why you came back -"
"Shut up, Merlin."
"- anyway, once we defeat the threat and don't die trying - I'm looking at you, Arthur, Gwaine, Elyan, Lancelot - Percival, you died fighting in World War Two, don't think you get out of this - and Leon, what were you even doing in the Battle of Waterloo? You weren't a soldier -"
Arthur coughs pointedly, and Merlin abruptly changes course.
"So I'm mortal, now, like you." Snatching a stray juice box from the kitchen counter, Merlin stabs the straw through the hole and takes a sip. "Thank god," he says as an afterthought. "If I had to live through another war because of the dumb decisions that self-entitled people in positions of power make, I might go insane for real."
Recovering from the shock of Merlin's newfound mortality, Lancelot shakes his head and pipes up, "Merlin, I found you in a mental hospital diagnosed with insanity. Three times. In three different centuries."
"You can't prove that," Merlin says stubbornly, and Morgana snickers, the stifling solemnity of the atmosphere easing.
"I was your therapist."
"It was the seventeenth century!" protests Merlin. "Therapists weren't invented yet!"
Arthur blinks, holding up his hands. "Wait," he says. "How many times were you admitted to a mental hospital -?"
Merlin rounds on him, narrowing his eyes. "You try living through fifteen hundred years of everyone you know and love ageing and dying around you, see what state that leaves your mental health in -"
Arthur finds Merlin again that evening, standing on the balcony and staring silently out at the now-silent city. Wordlessly, he approaches him, closing the door behind him and stopping a few feet away, unsure if Merlin was in the mood for talking.
"Hey," he says, after a few moments. Merlin makes a small noise of acknowledgement, but doesn't move, continuing to stare at the darkening streets. It doesn't take long for Arthur to pinpoint the strange expression on Merlin's face - sadness.
"What's wrong?" he offers, somewhat awkwardly. "I thought - I thought you were happy. Not being immortal anymore, I meant."
He had seemed elated a few hours ago - even though Arthur still couldn't bring himself to fully understand. The thought of Merlin dying - any thought of Merlin dying - still terrifies him, but he could see why the promise of an end to hundreds of lifetimes of suffering freed him.
"I am," Merlin sighs heavily. "God knows I am. But - it's just ..." he makes a half-formed gesture with his hands, trying to explain something Arthur couldn't understand. "The people I lived alongside, their memories, their stories ... I'm the only one left, now, who knew who they truly were. When I die, who's going to remember them? Ten years ago, a hundred years ago, a thousand years ago - once I'm no longer around to keep them alive inside me, they're going to disappear." He shrugs, a bitter, wistful kind of smile on his face. "I guess I just feel guilty. They all fought so hard to live for just another day, and I'm celebrating the fact that my days are numbered."
I'm really not qualified for this, Arthur thinks again, but he steps closer anyway.
"Don't be," he says gently. "Nobody should have to shoulder a burden that heavy. They lived, and died, as humans do, and whether or not someone alive remains who remembers them, the fact that they have existed in this world will not change, ever."
Merlin looks at him, properly looks at him, and gives him a watery smile, and the distressing, guilty feeling twists in Arthur's stomach again. I died, and left him. Left him to exist, all this time, without anyone who really, truly knew everything, anyone who understood him, understood his pain and the memories he's had to live with -
"Don't tell anyone I did this," Arthur huffs, and pulls Merlin into a hug. Merlin clings onto him like a dying man to the last shreds of life, and Arthur doesn't say anything about the silent tears he can feel wetting his shoulder.
"You know," he murmurs a while later. "Guess what you have to think about now?"
"Hm?"
"Retirement plans."
Merlin laughs a little into Arthur's shoulder. "Come on."
"No, really, I'm serious. If we save the world, they'll probably be nice to us for the rest of our lives, right? When we get old, we could choose any retirement home we wanted. Think about it - all of us in the same one, the knights and us and Gwen and Morgana, sitting in sun chairs outside, looking back at old photos and telling old stories as grandpas and grandmas -"
"Stop," huffs Merlin, and grins despite his words, pulling back a fraction, just enough to lightly hit Arthur's arm. "Now I'm imagining all of you with wrinkles and walking sticks."
"How much would you bet that Gwaine would still train his sword-fighting skills using his walking stick?"
"Please, he's definitely going to be the beer grandpa. More like Leon or Lancelot who'd still practice."
Their conversation strays into nonsensical, familiar banter, and Arthur thinks, this is enough. Whatever might lie ahead, whatever we'll have to face, if we still have this, it'll be enough.
Together, all of us, we'll be able to do anything.