1. Battle for Camelot (6th century)
It's been several years since Arthur died. Merlin has built a little shack for himself next to the lake, and he likes to spend most of his days there, reading and practising magic. He wants to be as close as possible when Arthur comes back to life.
He returned to Camelot briefly after Morgana and Arthur's death. He told Gaius what happened, could not bear to look into Gwen's eyes and hearing about Gwaine's death was the last drop. He packed his things and left with no intention of returning any time soon.
He's not a complete hermit. Sometimes, he gets visitors, which is nice. He corresponds with Gaius until he dies a few years later of old age. Merlin goes to the funeral, but comes back on the very same day just in case. Of course, nothing happens.
He has days when he waits patiently, almost numb to the pain. He has days when he does nothing but cries and begs every force he can think of to give Arthur back.
The following years are not peaceful, but it is not until a couple decades later that, Camelot is once again in serious peril. The army they are facing is stronger than Morgana's and Guinevere, the one he considers to be his last living friend, sends for him, begs for his help. I can't go, he writes earnestly, I have to be here when Arthur wakes up. He will wake up now, he is needed, and we will both come and save Camelot.
2. Normans (11th century)
Arthur hasn't woken up. After the defeat of Camelot about five hundred years ago, Merlin finally accepted that he was not going to return for a long time.
Merlin ages differently. His body, he realises, is no longer bound to any sort of natural law. He can take on an appearance of a young boy or an old man. He always feels, and truly is, the latter. His mind ages, not quite like those of normal men and women, but still. Memories fade after decades, more after centuries.
When he realises that he can not recall what Gwaine's face looked like, exactly – he struggles, can't recall more than a blur with brown wavy hair – he is, for the first time in so, so long, terrified to the bone. He writes down everything he can remember about Arthur, sketches him sleeping, his lopsided smile he wore when he was teasing him about being an idiot, acquires colours that match the blue of his eyes and paints his face again and again, from every angle. He visits all places they used to go to refresh them in his mind and writes down tales of their adventures. He illustrates Excalibur, copies Gaius' old books, describes Morgana's story and Lancelot's death. He doesn't stop until he has recorded and refreshed everything in his mind. He locks it all in a safe place, and spends the next few decades searching for a cure. A cure, he calls it, because the worst illness he could imagine befalling him is forgetting Arthur. He doesn't believe he could, not really, not ever, but then again, he can not recall how Arthur liked his eggs, and it's a small and silly detail, but the loss of it makes him feel sick to his stomach.
He's out of touch with the world until he figures it out, and that takes a long time. He burns the memories in his mind with powerful magic, makes them indelible until the end of time, and smiles.
When the invasion comes, he waits every day, patiently, by the lake. He gets his hopes up, dangerously high after years of resignation. He stays by Arthur's side and doesn't even seriously consider leaving to fight for his country. There is no one else he truly cares about left to protect.
Nobody calls for him this time. Everybody he once loved lies long dead.
3. English Civil War (1642–1651)
It's been over a thousand years.
He hasn't memorised anything after his time with Arthur properly, so seasons and years blur together. At some point, he feels like he could be using his time better, making an impact on the world, help people. He genuinely tries, but when he starts getting attached he comes to the painful realisation that he is going to have to watch everyone he ever helped die. He'll outlive everyone, and loving more people just meant more pain.
He retreats to his shack again.
His vain hope that thousand years would be enough was crushed. He spends that anniversary of Arthur's death awake by the lake, barely moving.
He feels empty and resigned, not even bothering with his studies or basic hygiene, merely replaying the too-vivid, too-real memories of Arthur and Camelot in his head until Civil War comes.
When nothing happens yet again, he considers the idea that he has to wake Arthur up instead of it happening naturally without his intervention when time is right. He's thought about it before, but never acted enough on it. He uses every method he can think of, but no matter how hard he tries, it only brings bitter disappointment instead of Arthur.
4. First World War (1914-1918)
It's getting close to fifteen hundred now. He reminisces over his life sometimes, and he has to admit there were some happy moments, but it mostly feels like a string of continuous disappointments since he lost Arthur.
This is it, he thinks in 1914, this has to be it. He shivers in anticipation as he prepares a British uniform in Arthur's size, acquires weapons – a selection, he'd have to show them all to Arthur, train him and let him pick his favourite - and stocks up on his favourite meals. He readies himself, reads up on the political situation in more detail than he has in centuries and waits.
He burns everything four year later, and if he screams in frustration, there's no one around to hear.
5. Second World War (1939-1945)
Merlin gives up counting years, he gives up guessing. He's done it countless times before, thought he could give up on Arthur, after all he only knew him for a few years and then lived for more than a thousand without him. He wishes he had not done the memory spell. He wishes he could let go.
He knows, without a doubt, that in hundred years, the memory of Arthur's first words will still make him chuckle. He knows that in a thousand, his last will still bring tears to his eyes.
Even when the Second World War begins, Merlin can't bring himself to truly hope again. For the last time, he promises himself, he waits by the lake. He doesn't interfere. Part of him, the part he detests the most, doesn't even feel bad for what people are going through. He's seen so many wars, so much pain and torture, and it all just seems to repeat on an endless loop, violence, suffering, greed, and he's always alone. The sick, unthinkable things people do to each other just make him wonder. What kind of terror will warrant Arthur's return, if this is not enough?
He shouldn't want it to come, but he does.
+ 1. 2089
He leaves Britain after the war, in the fifties. He travels the world, he meets new people, and forgets them soon after. He finds no magic.
Turns out that there is no real happiness of even vague contentment for him without Arthur. He goes back to Avalon, which has barely changed in his absence. He sits by the lake and tells Arthur all about the Great Wall of China and how long it takes to cross an ocean.
Merlin spends the Second Millennium New Year's Eve like he probably spent the first, a thousand years earlier. Alone, dipping his toes in the cold water of the lake of Avalon, waiting.
Bad things happen in the 21st century. There are wars, terrorist attacks, deadly diseases, terrible injustice. Nothing new, nothing that strikes him as incomparably worse to anything that has happened in his lifetime. Maybe that's why he doesn't feel it when Arthur wakes up. He doesn't even expect it.
It's a crisp morning in spring, appropriately enough, and Merlin's cleaning up some litter that kids left there a couple days earlier, partying or camping. It's not his land, not really, but it still feels like they're trespassing on something sacred. They all think he's a grumpy old man who lives nearby, which isn't too far from the truth, and he couldn't really care less anyway. He grumbles to himself as he picks up a can and places it in his black bag, when he hears a cough behind him.
“Yes, yes, what do you want?” he says impatiently, not even bothering to turn around. If another tourist managed to get lost, they can go drown in the lake for all he cares.
“What... Where am I?”
Merlin registers the old English, and he would know that voice anywhere, and yet what surprises him the most is the lack of feeling. His mind just completely refuses, against all evidence, to process the input as it should. His brain is fuzzy and suddenly, he realises he's dropped the bag and has literally forgotten to breathe, so he inhales sharply. He can not bring himself to turn around. He hasn't seen him in hundreds and hundreds of years, but to not see him in the very next moment would be the worst of all.
“What time is it?” the voice continues, and it sounds scared and unsure, which are two things Arthur should never sound like. Merlin decides to get over himself and turns around.
The sight takes his breath away again, and he forgets that he was supposed to answer Arthur's questions. He's standing only a few metres away, blonde hair wet, and he's still in the lake from the waist down. Merlin rushes towards him on instinct, but remembers his disguise when Arthur flinches in obvious fear. He transforms into his younger body, as close to what he looked like when they last spoke, and Arthur's face flashes in recognition. He brightens, and they both take the few remaining steps towards each other. Arthur is stark naked, and Merlin doesn't even notice. He's grinning like a madman, crying too, as he cups Arthur's face between his hands and leans in until their foreheads touch.
“It's the beginning,” he says.