Disclaimer: I own nothing, not BBC's Merlin nor any references. More on this on the final notes down below, since it'd be spoilery for your smart brains.
Also, not written as slash, but I've always thought all MerlinArthur fics were akin to a glass of water half filled/empty. It's a question of perspective, so take it the way you want.
Sometimes it was right in the middle of the best years of his lifetime, that unreliable line between youth and old.
Sometimes it was when his joints ached and his white beard was so long it could almost touch the ground.
Once, it happened when he was younger by a few years.
Sometimes, he chanced upon him, a helpful stranger in his way and he'd stay and chat.
Sometimes, he crashed upon his home, an odd man with eccentric ways and he'd be subtly forced to stay.
Once, it happened by accident, an insult bred a fight and both were tied together.
Sometimes, his magic was mostly gossips, fire, cleverness, dreams, logic and words from gods.
Sometimes, his magic was sparks, songs, lessons in water and trees and one slightly grumpy owl.
Once, he became magic, golden in nature and outlawed.
This time, he was old enough for grey to streak his hair but not old enough for his beard to be long.
This time, he came to him full of curiosity, having escaped his caretakers' vigilant eyes. He couldn't have been more than twelve.
This time, his magic was grass and whistles. Words in the wind and faces in the water. Knowledge and love.
Merlin's heart skipped a beat as he heard the joyous laughter from young Arthur. He couldn't keep his loyalty and devotion and love from soaring high at the sound. He knew this Arthur wasn't his Arthur, but all of them shared something and this laugh was his.
Still, this Arthur was a living Arthur and so Merlin would stay by his side, protecting him and advising him but, most of all, secretly hoping. Incessantly waiting for the "You're an idiot, Merlin" that would never come.
And when this Arthur died, Merlin would sat by the fire and cry, watching his memories from all Arthurs flash in the flames, watching as the memories from his Arthur burned brighter than all of them together. He would cry himself to sleep and dawn would always find him in front of the Crystal Cave.
He didn't know if he brought it with him or if the cave existed in all the worlds. Either way, it was always both his starting point and his final destination, for he would walk world after world, until he could find again his Arthur, the very first one, the one from the world where magic was gold.
A/N: I probably just took all the Arthurian legends and made an AU from them. Can't honestly say since all the legit literature I've ever read on it is about half Mary Stewart's Merlin series (give me a month and I'll probably double this amount), so noticeable references are either for her books or Disney's The sword in the stone (oh god, I dared). And BBC!Merlin, obviously.
Feedback is much appreciated.