First time Arthur woke up from sleep, half out of his mind with clawing , all consuming fear, he was 13 years old! He saw death and destruction, heard clashes of swords and roars of a dragon , he smelled blood in the air and tasted ash on his tongue, and then he felt a sword going through him, sharp and quick. Even after waking up with his breath caught in his chest and a scream in his throat, he could swear he could still feel the wound that was supposed to be fatal but wasn't, because he was still alive.
The nightmares didn't stop after that. The time when his friends dreamt of long slender legs, and heavy bosoms, of skimpy skirts and deep necked shirts showing cleavage, the time when all his age mates seem to talk about was girls and sex, was the time when Arthur started dreaming of blood. He saw himself, but much much older, wielding a blade and fighting off enemies, some human, some not; saw himself wearing chain mail and holding a shield, heard screams of men and women and children, heard his own voice commanding armies, and saw red: red of blood and red of fire and red of the knights under his command.
He dreamt of his father, Uther, tall and graceful, sitting on a throne and people kneeling in front of him; he dreamt of his sister, Morgana, sometimes being the regal princess she acted like in life too, and sometimes looking demented and haunted, and not like his Morgana at all. He saw Gwaine, with his bangs and his charm,Lance with his kind smile, but they starred as knights in his dreams, and he didn't understand why he still woke up crying from those dreams, until he saw them all die, slow and painful.
And he also dreamt of this one face, a face he was sure he had never seen before in his life, but was still so familiar that it felt like he had known him for centuries, a voice that seemed like the best friend he never had, and a cheeky grin that was the only relief from all these nightmares. It was when he dreamt of this young man, Merlin, that he slept almost peacefully, waking up with a smile on his face.
When his father caught him sitting in a corner of his room, shivering, too scared to go to sleep again because the nightmares would return, he called a whole team of psychiatrists and psychologists. They all analysed him and made him retell everything, broke it down into pieces and decided that he needed a therapist. They said it was just a teenage phase, because what child didn't dream of his father as a king and his sibling as an evil sorcerer.
What Arthur could never make them understand was this: what he saw couldn't be just dreams because sometimes even when he was awake, he could feel the weight of a sword in his arm, or the adrenaline of a fight. He could swear he still sometimes felt an arm around his shoulder, could hear a voice saying "prat! You're a prat, sire!" . Dreams didn't explain why he still felt the weight of an entire kingdom on his shoulders sometimes, why he knew people he had never seen before, how when Percival's family shifted to the house next door, he didn't need to be told his name.
No, they couldn't be just dreams, they were more than that: things that once were, or things that will come to be.
But he did realize with time that telling that to his therapists didn't work, and after his father sacked the third one in that many years, he figured out lying by omission was in his best interests.
He took up sword-fight training ,horse riding and fencing as a way to connect with his past. Lance and Gwaine followed him but always mocked him for his untiring dedication to being perfect at it! Once he trained so hard that he pulled a back muscle and was recommended bed rest for a couple of weeks, and when Gwaine found him on a the training field the next day, sweating and delusional yet wielding the sword like it was an extension of his arm, going through the moves like it was his second nature, he had to forcibly drag him back to his house!
"Stop being a bloody princess, Arthur, you would be no good to us dead!" Gwaine was saying, but he could hear a voice much older than that and much much more grave even with the hint of teasing behind it.
"Arthur, please take care of yourself, we are all counting on you," Lancelot was saying, his always kind and self sacrificing knight, the one who died because he was too loyal.
Arthur knew he was mumbling nonsense but he couldn't seem to stop. He was feverish, with past and present were mixing in together, and even though he was sure this would land him in trouble,he couldn't help crying and laughing at the same time because Gwaine and Lance were dead, but they were also so very very alive.
The dreams/nightmares/memories whatever one called call them, eased up a bit by the time he started high school; he assumed that was because he had figured out almost all the pieces of the puzzle that was his life centuries ago and he had made peace with the fact that he would have to live with past and present bleeding into each other. Like that time in junior year , when he sat next to a dark skinned pretty girl and called her Guinevere before she even introduced herself. Later when he saw her falling fast and hard for Lancelot, and realized that was how it was meant to be and didn't feel more than a twinge of regret at a love lost. Gwen was his in another lifetime, but she did always belong more with Lance. Like the fact that he was still terrified of raised eyebrow of Gaius'(his biology teacher), or like when he met a new freshman, when he was in senior year, his mind whispered "Mordred" and he froze in panic and dread, and dreamt of his death for the next entire week.
Life was as normal as it could be when you were a reincarnation of a Medieval Prince, and he had accepted the fact that he would always know the body count of old medieval battles, and the cultures of that time, better than text books ever could say, until a couple of weeks after his graduation ceremony, everything changed.
He was sitting in a garden, near a lake. The place was vaguely familiar and had a surreal feeling to it, like he had spent decades here and at the same time had never been here in his life. He felt calm , like time had slowed down , and he was sitting with his back to a tree, listless, waiting for something but knowing it was going to be a long wait.
Suddenly he noticed something swimming on the surface of the lake near him. Stretching out to pick it, Arthur realized it was a scroll of paper, not even slightly soggy or wrinkled even though it had been on the surface of the water since heaven knew when.
Unrolling it, he began to read, the writing achingly familiar,
"Everyone we have ever known is dead. Leon is dead. Percival is dead. Gwen is dead. Gaius is dead. That little girl that used to sing while carrying water home is dead as well. The young boy who used to look at you practicing, vowing he would become a knight one day died too. Nobody that you and i saw together, is alive anymore... but I live on! So I ran away from civilization ( I know what you're gonna say to that! You have always been scared of silly things, like being alone Merlin, why did you think it was a good idea), but I couldn't see people dying anymore. And when the silence drove me crazy, I decided to do what crazy people do. Write a letter to someone who would never ever read it because he is DEAD AS WELL! Why did you have to die Arthur, why? "
Arthur sat up straight in his bed, his eyes wet, his heart beating like mad, shivering. This, he was sure, was a memory too... not a dream.
The next day he packed his bag, told his father and friends he is going out of the country for a year to figure himself out, find himself, and went on a journey away from everything and everyone he knew. He understood that this first letter was just the beginning , there were going to be centuries worth of letters to read and he didn't fancy living through them with sedatives and antipsychotic medications and the world telling him he had gone insane.