Notes:
I needed to write this, desperately. I needed to work out why Merlin waited, and what he was waiting for. I needed to work out why, after all this time, Arthur had still not returned. And I needed to fix them, to save them from an eternity apart.
I hope reading this helps you heal, the way writing it did for me.
NOTE: The years are not actual dates. They chronicle the amount of time that has passed since the day Arthur died.
Day 1
Stay with me.
Merlin had begged, desperate, sobbing Arthur's name. But Arthur hadn't stayed, hadn't been able to, even though he'd tried. Even though when Merlin had shouted his name, Arthur had pulled himself back from the abyss for the briefest of moments, chasing the sound of Merlin's voice.
Merlin could still feel Arthur's hand, growing weaker by the second, stroking the back of his head. Holding on.
The tears would not stop falling.
Day 2
Percival found him still sitting by the lake, staring out towards the island where Arthur's body had faded away, out of the land of the living.
"The king is dead," someone said, and after a moment Merlin realized that it had been himself. His voice was raw from disuse.
Percival had fallen to his knees and wept, but Merlin's eyes had not left the lake. Percival was alive and well right now, but to Merlin, he was nothing more than a shadow, already swept away by the cruel tide of time. Already a memory, quickly fading, even as his warm hand came to rest on Merlin's shoulder.
He did not cry when Percival told him that Gwaine was dead, because to Merlin, they were all already dead and long gone.
When Albion's need is greatest, Arthur will rise again.
And Merlin would be waiting.
Day 3
Merlin did not go with Percival, even when the knight begged him to.
Camelot was where Arthur had lived. But now, Arthur was dead. Merlin had no business there anymore.
Merlin wondered if a part of him had died when Arthur had, and if what was left of him was only a ghost. He was breathing, and his heart was beating, but he did not feel alive. If he returned to Camelot, the others would tell him it was grief; that it was painful, but would pass in time. But Merlin knew better.
It wasn't him who was a ghost. It was everyone else. Already names in the pages of history books, the details long forgotten.
Guinevere's smile. Gaius' waiting meal, which Merlin would never eat. Gwaine's laugh. Morgana's twisted mind, broken beyond repair, only brought to peace when her body was penetrated by steel and left cold and broken on the ground. All of it was as insubstantial to Merlin as specks of dust in the breeze.
All that existed now was time, endless, stretching out ahead. And the promise of Arthur.
Year 1
Just hold me.
Sometimes the memories came to him unbidden. Arthur's final moments would assault him, everything he should have said leaving him wrecked with grief.
These were the moments when Merlin felt most alive.
He never wanted to forget.
Year 3
The news that Guinevere had married Sir Leon did not surprise him. He wasn't sure surprise was even an emotion he was still capable of feeling.
The small surge of relief he did feel reminded him that he still felt anything at all.
He smiled. Gwen was already lost to Merlin, but at least she would not live out her days alone. Loneliness was a burden Merlin wouldn't wish on anyone.
Year 5
Gaius had passed away.
Merlin did not cry. To Merlin, Gaius was already long dead.
Still, an image of a supper left untouched on the table came unbidden to his mind, and his heart ached with all the missed opportunities, all the what ifs of his life.
But in the end, all of this would be nothing but memories. None of it mattered.
Year 10
Merlin stood at the Lake of Avalon. Waiting.
It only just occurred to him then that he had not aged a day.
I don't want you to change.
"When does Albion need you, Arthur?" Merlin breathed into the silence. "What about what I need?"
He received no reply.
Year 17
Sometimes Merlin would wake up thinking he was back in his old chambers, and that any moment Gaius would knock on his door telling him he was late again.
Sometimes Merlin could still hear Arthur calling his name.
Sometimes Merlin could feel Arthur's lips on his own - but he rather thought this was the result of a too frequent dream in the moments between wake and sleep.
Sometimes, Merlin wondered what the difference was between dreams and memories. Or if it even mattered, if he was the only one left who knew which were which.
Year 34
"The Queen is dead. Long live the King!"
Merlin saw Camelot in ruins even when he knew it still stood, now to be ruled by Guinevere's young son. It was of no consequence.
He was living in a long-forgotten age, pulled out of time and memory. It didn't matter what was now, and then, who was dead and who was alive.
They were all dead, gone, forgotten. Yet they all lived, as Merlin remembered them, smiling and breathing and oh so very much alive.
Merlin never did go back to Camelot.
Year 100
It was all that remained of Arthur, this sigil which Merlin still carried in his pocket.
Sometimes it was all that reminded him that Arthur had ever lived at all.
Year 176
Merlin let his beard grow out and his skin turn wrinkly and grey.
He wandered. But he always came back to Avalon, where his king was sleeping.
Thank you.
Year 350
One day, Merlin woke up and wondered if, had the roles been reversed, Arthur would wait for him.
Merlin wondered why he did wait. Was it destiny? Servitude? Loyalty?
A flash of memory, as if from a half forgotten dream, showed him a tender smile across a camp fire, let him feel the ghost of a hand on his shoulder, a desperate "Where have you been?" And he knew that, for what it was worth, those memories were definitely real.
It was love which kept him waiting.
And Merlin finally knew what he must do. He must help the world remember Arthur, in all of his glory and for all the reasons why Merlin had loved him. He would make the Once and Future King the legend he deserved to be.
Year 623
Merlin had seen love, in all forms.
He had seen the familiar, comfortable way in which Arthur had loved Gwen.
He had seen passionate, desperate embraces as worlds collided around lovers' ears and they were powerless to stop it.
He had seen make-believe that looked more powerful than anything real, and had to remind himself not to believe it.
All love was fleeting. It faded with time, an echo of a smile or a ghost of a touch, sweetening the world infinitesimally but also bringing with it so much pain.
He knew that, should he ever tell anyone who he was and why he was still waiting, they would proclaim his story one of tragedy and love and loss and beauty.
And maybe it was. But no one would ever know, so it was meaningless. Stories only became real if others believed them to be, Merlin knew that now.
Arthur, as Merlin remembered him, was no longer real. Merlin, as he had been with Arthur, was no longer real.
It struck him now that he could make it all real. He could weave his own story into the texts, he could make millions believe it. He could make his own story anything he wanted it to be, anything his foolish young self had dared to wish it might have been.
But he never would, because that would mean that the story was over. And so this would be his and his alone: a story forever untold, the love story of Merlin and Arthur.
Year 1000
People remembered. But Merlin was beginning to forget.
Only at Avalon's shores did the image of Arthur as he had really been swim into his mind, and Merlin would return there, let himself close his eyes, and remember everything: the harsh smell of blood and death, Arthur's blue eyes holding his for as long as he could, the final, brief tug of his head down towards Arthur's which could have meant anything but which Merlin thought might have meant everything.
This time, he returned to Avalon to find that the lake had nearly dried out.
"No," he whispered, feeling the magic seep out of the lake, out of the land. He had known that magic was slipping away, into the earth, waiting to be called upon again but somehow he never thought it would happen here. "No. Arthur."
Magic was leaving this world. Surely this was when Albion most needed its king. Surely this was when Arthur would return.
But Arthur did not come.
The dragon had not known about this, Merlin realized, falling to his knees. Once the lake dried up, Arthur would be lost forever.
No.
Merlin brought out his water skin, and stuck his hand into the lake.
The water lapped around his skin, clinging to his magic. It felt like fingertips were sliding against his own, and a renewed sense of hope filled him for a moment.
It was enough.
Merlin sealed the skin with magic and put it back in his satchel.
Year 1345
Sometimes Merlin wasn't sure what were his memories and what were his dreams.
When he woke up with the feeling of Arthur's hand in his hair, or his own arms around Arthur's body, Merlin did not know if it had been a friend's goodbye or a lover's caress.
He was struggling to remember why it should matter, if it really made a difference. If anything was more real than anything else, considering that none of it existed outside of his own head.
Merlin remembered Arthur calling him friend. He remembered little, insignificant things like polishing armour, and sometimes huge, life altering things like Arthur offering his own life for Merlin's, asking robbers to spare his life, always coming back for him.
Arthur would always come back for Merlin.
Wouldn't he?
Year 1603
The history books had got it all wrong.
Or maybe it was Merlin who misremembered.
He wasn't sure what parts he needed to correct, or if he even should.
Maybe the real King Arthur, Queen Guinevere and Sir Lancelot were only figments of a mad old man's imagination.
Maybe the ones the storybooks spoke of were more real, now. The King Arthur of legend lived on in millions of people's minds, inspiring them to do great deeds, the stories painting a glorious past and the hope of a better future in the minds of men. That version of Arthur was truly the immortal one, transcending time and space, an eternal symbol, an everlasting promise.
The real King Arthur had lived for 30 short years, in a long forgotten age. He had lived like any other man, and died like any other man, of a wound, in a field, alone but for his best friend, the magic to his courage.
The crinkle of his eyes when he smiled, the love he held for his wife and his men, the way he said Merlin's name - those things were all forgotten, except by the old man wandering the world alone, the last relic of this forgotten time.
Only the fact that he was still here was proof that any of them had ever really lived at all.
Year 2000
There were a lot of times when Merlin had thought Arthur would be called back.
Empires rising and falling.
Wars. So many wars.
Death. Birth. Birth. Death. An endless cycle, lives passing Merlin by in the blink of an eye.
But Merlin was not a part of this world, not really. He was only one half of a whole, the other half resting, waiting, pulling Merlin back and holding him tight, making him feel foreign to this ever-changing world while keeping him tethered to it.
Time meant nothing. It was only a distraction, until Merlin was made whole again.
But maybe it would never happen. Maybe Albion's time had passed the world by, and Merlin was waiting for an Arthur who only now lived in the pages of books.
Year 2255
Sometimes Merlin wanted the history books to be right.
Arthur had, after all, just been a man. A flawed, stubborn, ignorant man, like any other. What had made him special after all these years was only what others had imagined him to be, it was the idea of him which lingered and inspired.
Merlin was left to remind the world of who Arthur had really been.
Bu what if Merlin had forgotten?
Would that make the Arthur from the stories more real than Arthur himself?
Would that be a good thing?
Year 2768
Arthur.
If he would forget all other words, Merlin thought that this one would always ring in his mind, lost and alone in the darkness.
He could not remember what Morgana had looked like. He had forgotten which of the knights had died in the Dark Tower.
He wasn't sure why the name "Gawain" looked so wrong when he saw it written down.
He did not remember if he had appeared young to Arthur like his more and more infrequent dreams were telling him, or if the history books were right and he had always looked like this.
He had not used magic for centuries.
Merlin was, for all intents and purposes, only a very old man who lived for the visions of a life long forgotten, a destiny long lost. Keeping the memory alive of a man he himself had almost forgotten.
Only the water skin in his satchel reminded him that there was a future for him, that he was still needed.
Year 3000
In a way, King Arthur had already returned a hundred times over.
Merlin watched as authors captured him in the pages of their stories, as actors interpreted his words and made them their own.
He watched generations be inspired, and realised that maybe this had always been what Albion needed. Not the actual man, just the echoes of him, reflected in all men and reminding them what it means to be strong and brave.
Maybe Merlin's mission was to make sure that people didn't forget the legend.
Maybe the only one who still needed the man was Merlin himself.
Now
It was a truly unremarkable year.
Leaves were turning shades of reds and browns and yellows, and they crushed under Merlin's feet, already dead and fading from the world.
It might have been a Tuesday.
On this unremarkable day, just another to be lived by all who had the luxury of living, Merlin's feet carried him back to where he would always return, no matter how far away his travels took him.
Avalon stood as it ever had. Dry and empty and devoid of any magic which had ever resided within. The Lady was gone.
Merlin took a deep breath in, feeling his lungs fill up with the air of the place where Arthur had died in his arms.
This was the last place where Merlin had ever held Arthur. The last place he had ever uttered his name out loud.
When Albion's need is greatest, Arthur will rise again.
But Albion was no more. Merlin finally understood that. Arthur would never return; his brief life was nothing but a flash of the brightest light in an eternity of darkness, echoes of him like the gentle pulsating of a dying sun.
The world had seen countless of unimaginable horrors. The British kingdom had risen to greatness, then dwindled to an island of half-forgotten traditions and nostalgia for a time long lost.
Merlin could no longer remember if there had ever been a time when Arthur had been anything but a shadow of a thought in his mind, when Arthur had been tangible and touchable and real.
And he could not imagine that Arthur would ever become so again.
Arthur would never return.
Merlin opened his satchel and took out his water skin, looking at it wistfully. Touching his fingers to the tip, wondering if Arthur's lips had ever really been wrapped around it like he imagined them to be.
That might have been another dream.
Merlin took the sigil out of his pocket, laying it on his palm and tracing the outline of the little bird motif with his thumb.
He remembered, absurdly, what grief and anger Ygraine's death had brought to Uther, and the years of terror he had wrought upon the land because of it. He remembered how huge of an impact Merlin had thought this had had on his own life, and how much pain so many souls had endured because of one man losing his wife.
Now, Uther and Ygraine were nothing but thoughts in Merlin's mind, at the mercy of his memory.
He regarded the sigil. Such a small trinket carried so much inside it - memories of hands sliding over it, promises made as it was passed on. Just take it.
Just hold me.
"How time makes fools of us all," Merlin whispered into the silence.
It was done.
Merlin let the sigil slip from his fingers and heard it land softly on the grass, grass which drew strength from an earth no longer filled with magic.
He took the cap off the water skin and felt the threads of magic he'd used to seal it weave tentatively into the air, the shock of pure magic against the stark emptiness of a world without it making Merlin gasp. It was like plunging into a stream of ice cold water.
It had been a while since Merlin had felt anything at all.
Merlin upended the skin, watching the water flow out in a mesmerising stream, wettening the patch of grass below and seeping into the earth, disappearing like the lake from which it had come.
When the skin was empty, he let that fall away, too. All magic had left it, and now, he knew, it would simply decay like anything else. As it should have done, thousands of years ago. As he should have done, had he not been tied to an insufferable clotpole.
Merlin snorted, despite himself.
"I'm not going to apologise for that either," he said, his voice echoing slightly in the light mists of autumn. "What, no witty comeback?"
He could imagine a comeback, of course. But he thought that might hurt more, knowing that Arthur's lips would never actually utter the words.
For a moment, Merlin stood still, staring down at the ground where he had let the last of Avalon's waters fall, hardly daring to believe what he'd done.
"You didn't come back," he murmured, defensively, to no one. "You're only a story now. Nobody needs you anymore."
A bark of laughter made Merlin jump. He hadn't heard anyone approach.
"Well, that is certainly a disappointment, Merlin. Here I was hoping that you still did."
Merlin's heart stopped beating, for one impossible moment.
That voice. Even after millennia, Merlin had never forgotten the way Arthur had said his name.
But it could not be.
Arthur.
Merlin didn't turn around, but stayed staring at Avalon. No, it wasn't happening. He had just finally lost his mind.
"Merlin. Merlin." The voice sounded fond, exasperated and teasing.
Merlin couldn't breathe, couldn't move.
He heard footsteps. He felt a body next to his own.
A hand touched his shoulder, and Merlin's knees gave way. He stumbled to the ground, his hand landing on the sigil, the metal boring into his palm.
Magic flooded his veins. He gasped, unable to breathe. It was like waking from a long sleep, but he couldn't, he wouldn't turn around.
He always thought he would feel it, when it happened. Surely it couldn't be now, here, of all times?
Merlin couldn't bring himself to look. He didn't want to be wrong.
"Merlin, gods," the voice was suddenly hesitant, concerned, but the body followed him down, hand never leaving his shoulder. "Are you alright? Merlin? You idiot, look at you, how long have you been waiting?"
Merlin turned his head and met startlingly blue eyes, saw cheekbones and lips and golden hair and it was nothing like the story books, nothing like the actors in the movies.
It was Arthur. It was really Arthur.
"It can't be you," he breathed out, "it can't be. It can't be you. Arthur."
He was shaking, he realized, but he grabbed Arthur's hand with his own and held it tight, his eyes never leaving Arthur's. They were blue, beautiful, and so very much alive. It couldn't be true. But oh, how he wanted to let himself believe it.
Because Arthur had slightly crooked teeth. Arthur had callused hands. Arthur's nostrils flared when he was angry.
Arthur was a man. Only a man. But he was here. He was real. He was now.
"You got so old," Arthur breathed, his voice sounding broken. "Gods Merlin, you idiot. I didn't ask you to wait for me."
Merlin's mouth fell open in mute shock, his hand tightening briefly around Arthur's as a familiar anger swept over him and he let go, stumbling back, falling onto the grass, emotions so strong he felt like he was breaking sweeping over him.
Arthur looked alarmed, and concerned, like he thought Merlin might break into a million pieces.
Merlin broke into a rather hysterical laughter. The magic, so familiar yet unfamiliar in his body, was pushing and pulling at him in a million directions at once, and somehow all directions were towards Arthur, but Merlin was scooting further away on the grass.
"Right," he said, "right, of course not. Why would I wait? That would be stupid. Stupid Merlin." He knew he was rambling, but he couldn't think, he couldn't make his mind comprehend what he was seeing before him. Arthur, just as he had been. Just as the picture in his mind which he hadn't even realised was still there. "You were only a man. You are not your legend. A legend I helped tell. You are just... just..." He trailed off, and to his horror he felt something wet slide down his cheek.
He had not spoken this much in years and his voice felt hoarse, and he remembered desperate words and promises and caresses and realized that none of them were real, that only he had caressed and promised and that he had waited all this time for something which had never really existed, and it crushed him.
Arthur stood before him, looking lost and broken and a little like Merlin had felt on the day when Arthur had died. Like he was the one who'd lost something.
And Merlin loved him, because he was here and he was real - and hated him, because Arthur had not come back for him. And now he finally was, after all this time that suddenly felt like no time at all.
But this was just a moment, nothing like his dreams, nothing he could take and shape and create into what he needed it to be. It was just Arthur. It was Arthur. It was really Arthur, beautiful and breakable and tragically imperfect. It was the most wonderful feeling in the world, even as Merlin's heart was aching with pain and loss and heartbreak.
"I'm sorry I couldn't stay with you," Arthur said, so quietly Merlin almost missed it. "I'm sorry I couldn't come back. I was waiting for your call."
Merlin blinked. "No," he found himself saying, shaking his head, feeling his long hair move over his hollowed cheeks. "No, I needed you. I called you, Arthur. You didn't come. You didn't come back."
"You kept my memory alive," Arthur said, still crouched down and looking at Merlin like he was a frightened animal. Merlin had a flash of memory, seeing that same Arthur crouch down while on one of their many hunts. Just a small, insignificant moment in time. Lost and found again and Merlin had not been prepared for this feeling. "You never forgot me. You never needed me, to complete our destiny, only the memory of me, Merlin. But now you have forgotten yourself." He looked pained, and oh so young, even as he spoke words that sounded like wisdom from a much older soul.
I don't want you to change.
"I was never myself without you," Merlin breathed, and it sounded like something out of a storybook, something made up; not a stark, imperfect reality like this one.
Arthur snorted, the movement lighting up his face and highlighting a spot of dirt on his chin. Merlin made an aborted movement with his hand, to reach out and feel, to know that this was not a mirage, not a charade, or a picture. He had forgotten what reality felt like, but it was beginning to flood his senses again. Smell and touch and quickening heartbeats.
Arthur caught and held his hand. "I wanted to come back for you."
Merlin looked down at their joined hands. Where Arthur's was young, golden and smooth, Merlin's was grey and ancient.
"How long was it, for you?" he asked, his voice hollow and worn with countless lifetimes.
Arthur was silent for a bit before he answered. "I don't know. A long time. But also, no time at all. Your eyes were the last thing I saw, and it feels like I never looked away."
Arthur didn't sound abashed or hesitant about his words. Merlin supposed their final goodbye, fresh in Arthur's mind, may have wiped all the pretense away about how strongly they had felt for each other. It certainly had for Merlin.
He realised that maybe none of it had been imaginary at all.
An ache of long-suppressed grief which never really faded made him close his eyes against the onslaught of memories.
"Merlin," Arthur breathed, and was moving forward, and then his arms were around Merlin and hugging him so tight it was almost like Arthur had missed Merlin as much as Merlin had missed him.
And maybe he had.
"I'm so sorry," Arthur breathed, sounding as destroyed as Merlin felt, "I'm sorry that I left you alone. I'm sorry you had to wait for me. I'm sorry that I'm so glad you did."
I want you to always be you.
Merlin's arms closed around Arthur, and he was suddenly aware of how thin and veiny they were, of how carefully Arthur was holding him - of how Arthur was holding Emrys, not Merlin, but still seeing his true self within, that he didn't even care.
He realised in that moment that Arthur was truly back, that Merlin's wait was truly over. Indescribable relief and something he thought might have been happiness washed over him, and Merlin finally set the magic free, flood his veins, now knowing that it had been waiting, like him, for Arthur to return.
And he let the tendrils of magic stretch out, across the fields of what had once been Albion but now was only a collective memory helped along by a ghost of a king and the pen of a wizard, feeling his own magic seep into the earth and take root there, encompassing everything and yet leaving it be, having done everything it had needed to do.
When Arthur pulled away, his smile was like the sun, and like the moon, Merlin found himself reflecting it.
"Look at you," Arthur breathed, and he did, tracing a finger over Merlin's cheek and looking at him like he was something precious.
Merlin reached up to push the finger away, suddenly self-conscious, of all things, when he caught sight of his own hand.
Pale, and smooth, and such a beautiful contrast to the rough gold of Arthur's when the other man caught and held it with his own.
Merlin's eyes met Arthur's, whose own were flittering over Merlin's face as though mesmerized, looking like he was trying to memorize every crease and crevice of long-forgotten laughter and smiles. Or remembering everything, seeing Merlin now as he had seen him last: young and alive and with tears clouding his vision.
"Look at you," Arthur repeated breathlessly, "after all this time, you stayed. You waited for me." He sounded honoured, overwhelmed, and happier than Merlin had ever known him to be.
"Always, Arthur," he breathed, and this time when Arthur's hand went to his face, to trail a finger over his now-smooth cheekbone and ghost a touch over his ear, moving back to rest on his neck and twirl a strand of black hair around his fingers, Merlin didn't push Arthur away.
They had never quite been like this, Merlin realized, but it didn't matter now. Because they couldhave been, and now they had a second chance.
"So much I never got to tell you," Merlin whispered, and Arthur's gaze flickered to his mouth. "Things I should have said..."
"Shh," Arthur breathed, his head so close that Merlin felt his exhale - glorious proof that Arthur was breathing, alive, with the beautiful vulnerability and temporality of a mortal man - and then Arthur touched his lips to Merlin's.
It was brief, chaste, and tender with promise. And it felt like life, to Merlin, like everything he had seen around him for thousands of years; tragically beautiful specks of light illuminating the world and burning out like the stars in the night sky, which Merlin had not felt within himself since the day Arthur had passed out of this world and Merlin's own mortality had been whisked away to be replaced by emptiness.
It was the kiss of life, except it was the gift of death which Arthur was giving him. The gift of knowing that from now on, Merlin would age and die like any other man, like Arthur. And realising that only legends were meant to endure, that real lives were only beautiful because one day they would end, Merlin knew it was the greatest gift he could ever have been given.
He opened his mouth against Arthur's and let his tongue sweep over Arthur's bottom lip like he had done so many times in his dreams, except now it was real, now this moment would end and his memories would never do it justice again, and Merlin could have cried with relief and joy.
Arthur groaned and angled his head, his own tongue meeting Merlin's with such vigour, Merlin knew that he was not the only one who had imagined this before. There was no tentative gentleness now, now that Merlin was young again and they were both just as they had been, before their futures had been taken away from them.
Arthur broke away, panting, leaning his forehead against Merlin and it was like before, only this time it was not the end but the beginning. "I love you. I meant to say it, Merlin, always, I love you."
Now it was Merlin's time to shush him. "I know." He laughed. "Do you think I would have waited around for you all this time if you didn't? Clotpole."
Arthur's laugh was the last sound either of them would make for a long time, kneeling at the place where they had once said goodbye as friends, and where they would begin their journey together as lovers - as it always should have been, and always would be.
Storybooks would keep King Arthur and his wizened old magician Merlin alive in forms that would forever remind humanity of the greatness of man and magic.
Merlin and Arthur had given their lives to make these stories possible - Arthur by dying and Merlin by living on, and remembering.
Now they had done all they could for the world. And so they would live out their mortal lives, together, until they too would fade to specks of dust, and none would be left who remembered them.
And it wouldn't matter, because for one brief moment in time Merlin and Arthur's love had lit up the world like the brightest star in the sky, and even if their bodies and minds would be lost in the folds of time, the earth would remember. The magic of the world would wrap itself around that light and keep it safe, and Merlin and Arthur would never be parted again.
Reflections on the story:
I have been thinking a lot about what would make Arthur finally return, and I realised that in a way, he never left the world at all. King Arthur is the sum of all the stories about him, and he lives as long as we keep remembering him. As such, "when Albion's need is greatest" actually means, "when Merlin's need is greatest," because Merlin is the only tangible thing left of the long lost kingdom. So it was Merlin, ultimately, who had the power to call Arthur back, at the moment when he had no more story left to tell because humanity had already taken everything he had to give and twisted it to suit their needs.
The way I see it, the world no longer needs King Arthur in the flesh. In today's day and age, what we need more than anything else are stories that move and inspire us, legends we can aspire to live up to. The real thing is oftentimes a discouraging disappointment, compared with the memories and the dreams.
This was the price Merlin and Arthur had to pay. Arthur had to die because Merlin had to live on alone, forced to keep Arthur's memory alive in the minds of men. He influenced the stories that were written about King Arthur, he turned a fleeting life into an undying legend. Now, in present day, there is nothing left to say, and so Merlin's story is finished. His reward is Arthur's return; Arthur, who has been lying dormant all these years. In this way, really, Arthur doesn't return when Albion's need is greatest, he returns when Albion has no more need of his actual being, because the stories and legends have taken over, and are carrying on his legacy.
Now, Merlin and Arthur can live out the rest of their mortal lives, from the point at which they were both snatched out of time, in any way that they see fit, their destinies finally fulfilled. And they choose to do this together, of course, their bodies entwined now as their destinies have always been.
This is the end of the story. Believe it, because only your thoughts can make it real.