After Arthur's death at Camlann, Merlin returns to Camelot and finds the Horn of Cathbhadh. He does research this time and finds that it can be used to summon the same spirit more than once, but only if the summoner does not look back and bring the spirit into the mortal world. If the spirit is brought forth and then sent back, the door is closed forever.

Merlin gives Gwen the chance to see her husband one last time, but doesn't tell her that it can be used more than once. Gwen gets closure and thanks him for giving her the opportunity to say goodbye. Then Merlin summons Arthur. And Arthur tells him that he isn't angry, that he can see everything from where he is now, that he knows now what Merlin's truly sacrificed for him. He tells him that he's grateful and that he's never been blessed with a better or more loyal friend. He begs Merlin not to blame himself because he more than anyone deserves a chance to be happy, but Merlin knows he won't be able to manage that without Arthur.

Then Arthur tells Merlin that it isn't over, that he will return. He doesn't know when and he doesn't know how, but one day he will live again. And Merlin will be waiting for him. He says that he doesn't know whether to be selfishly grateful to know that he will have Merlin when he returns or to curse whatever powers may be for forcing Merlin to wait for him. He tells Merlin that the horn will summon him two more times, and only two, and to use them sparingly. He says he will be watching over Merlin, always with him. And please, whatever he does, don't look back.

It's three hundred years before Merlin uses the horn again and Camelot has long since fallen. Everyone he once knew is dead and the world is changing so drastically around him. Arthur hugs him and he can almost feel it, just a barely there pressure and a vague feeling of warmth and security. Merlin apologizes for summoning him, but he just needed to remind himself what he was waiting for. They talk about Merlin's experiences and all that he's seen, laughing over silly things and basking in each other's presence while they can. Arthur promises to watch over him, promises that he'll return someday, and warns Merlin not to look back.

It's another four hundred years before he summons Arthur's spirit again, and the years have not been kind to him. He has tried so hard to maintain his faith, to remember what he's fighting for, but he couldn't bear the loneliness and the grief and the guilt. His magic wanes until he hardly has enough to light a candle and the emptiness yawns inside of him, a black hole of failure and accusation. The first time he shoots up, he looks up at the sky and thinks of Arthur, wonders if Arthur's watching over him now. Then he wonders if Arthur's real, if he's ever been real. "I'm sorry, Arthur," he whispers, but he doesn't stop. He stays in that drug den for ten years. He overdoses several times, most of them accidental, a few not, but he lives. He always lives. By the time he's clean enough to remember the horn, he can't bear to use it, too ashamed of what he's done and the depths to which he's sunk in his weakness. But eventually, when he's convinced he's gone mad and that Arthur and Camelot were just a figure of his imagination conjured up to explain his unnatural lifespan, he blows the horn.

Arthur holds him tight, tight enough that Merlin can really feel it, and curses him through his own tears. Merlin sobs and clings to him, too overwhelmed to do anything but babble apologies into Arthur's shoulder. Arthur shushes him and strokes his back soothingly, calling him an idiot in the gentlest tone possible. "You're not crazy, idiot," he says, "I'm coming back for you. I am, I swear it."

"When?" Merlin asks, trembling. "I can't take it, Arthur, I can't. I can't live like this."

"That's because you're not living, Merlin. When's the last time you actually spoke to someone who wasn't selling you narcotics?" But how can Merlin interact with these people, these mortals whose lives are over so quickly? He can't make friends, can't form attachments, when he knows that he'll blink and they'll be dead like all the rest. Alone is better, he says, alone protects him. Besides, it's not like he could tell them the truth anyway. He would just end up in an asylum. He doesn't want to lie anymore. Arthur sighs and blinks back more tears at the sight of what has become of his friend, once so cheerful and exuberant, a friend to all regardless of the necessary deception.

"Talk to me, Merlin," Arthur says earnestly. "Please, whenever you need to. And just know that I'm listening, and that I'm talking back, whether you can hear me or not." He promises Merlin that the time is coming, that he's coming back soon. Of course, their sense of time is a little skewed, Merlin points out. Soon could well mean another five centuries. And this is the last time he will be able to see him before that day. They linger as long as they can. Merlin asks haltingly what would happen if he looked back, if he brought Arthur into the world with him. Would he stay with him? Arthur shakes his head sadly.

"You saw what happened to my father."

"You're a better man than him. Always have been."

"You don't want that to happen to me. That's no way to live, Merlin." Merlin nods, tears in his eyes. He'd known the answer before it was given, of course, but he'd had to ask. They embrace again and more promises are given on both sides. Don't look back, Arthur says, and Merlin doesn't.

Merlin tries after that, just a little bit more. He stays away from the drug dealers and ventures into the cities, throwing himself into studies and research. He learns languages and takes university courses and travels the world. He keeps up a running stream of commentary for Arthur whenever he can, reminding himself that Arthur's listening and imagining his sarcastic responses. It's not really a surprise when someone finally calls him a lunatic. He sits on the crisp white bed in the crisp white room and looks up at the ceiling.

"I told you I'd end up in a loony bin, Arthur. I told you." It is a relief in some ways to be able to speak freely, to not care if someone looked askance. There is a twitchy man named Jimmy who listens to all his stories and nods sagely, like it's all perfectly reasonable. "Talking to Arthur?" Jimmy would ask, pointing up at the ceiling. Merlin would nod. "Ah. That's nice. My Sheila won't leave me alone. She wants me to set fire to the reception desk." He would dig a finger into his ear and Merlin would nod and smile like it was perfectly reasonable. It was only polite to return the favor.

There are days when he is happy to have the attendants drug him into complacency, floating in a haze until he can't remember all the things they insist aren't true. Who is Arthur? Arthur is a delusion, of course he is, he isn't real. Schizophrenia, they say. Hallucinatory episodes, they say. Delusions of grandeur, they say. And when they say it often enough, sometimes he believes them. Until, of course, he's been there fourteen years and hasn't aged a day. Then he weans himself off their drugs, vanishing the pills with a blast of the magic that is slowly but surely returning to him, and disappears in the night, leaving behind a locked but empty room and several baffled attendants and security guards.

Fifteen hundred years after his death, Arthur comes walking out of the mists at Avalon, checking both ways for cars along the freeway before stepping into plain view and pulling his sorcerer into his arms, thumping him on the back and squeezing until Merlin can hardly breathe. They cling to each other, Merlin reveling in the warm, solid presence of his king and Arthur relishing the sight of Merlin whole and comparatively well before him. He pulls back and just looks for a moment, clasping his shoulders tightly. He shakes him once just to see Merlin's affronted expression then beams.

"Are you ready to live again, Merlin?" he asks.

"Took you long enough," he huffs.

They turn and walk away, and they don't look back.