In Arthur's dying breath, Merlin feels the world stop. He sends fingers of thought down into the wild earth (deeper than roots dream). Magic richer than minerals and blood wick upwards into his body, soak into him like sunlight, flicker golden in his eyes.
He loves Arthur more than the world, more than his heart can contain, and he must watch: Arthur on the cold muddy battlefield, blood leeching into the dark earth like ink into parchment, life ebbing away.
With everything he has, with every thought and whim and breath, Merlin screams into the void in his head and feels his voice echo over time and distance like thunder: "No. No, you cannot have him!"
Something pulses, bright, in the corner of his mind or perhaps the world, something shimmers pinprick in the grief he feels flooding his body like a force of nature. An echo, perhaps, or an answer:
"There must be balance."
And he surrenders to magic more powerful than his love, "Take it, take my magic, but give me him."
There is a peal of laughter, or a bell, when the voice that is the world responds, "Keep your magic. Your life shall satisfy."
And then silence, the kind that happens at dusk or at funerals, or at the beginning of time, the heartbeat of silence that precedes great action or a wail of misery or follows a deal wrought in blood and fear.
And then Merlin feels the life pull from him, like he is a great bowstring and an invisible finger is setting an arrow to him. It is the worst pain he has ever felt, his every breath he has ever taken or will take, past present future beyond, ripped from his chest, a thousand heartbeats he's never felt erased, and just before he wishes for death, before it crosses the boundary, it ceases.
He opens his eyes to a pale dawn, to Arthur, who, before his eyes, seems to withdraw all his spent blood, soaking backwards from the ground, coloring his pale cheeks. Arthur opens his eyes and inhales sharp iron-tinted air and Merlin knows that whatever has happened, whatever was traded for this, for Arthur, it is worth it a thousand times over.
But when Arthur's eyes focus on Merlin, his brow wrinkles in a ghost of confusion or recognition.
In the reflection of his eyes, Merlin knows what he sees, what shocks him.
Inside a face that has seen too much loss and death and a hundred years or more, inside a wrinkled mask and underneath far too much white hair, Merlin sees blue eyes awash in gold, eyes far too familiar for their aged setting.
"...Merlin?" Arthur reaches up, hand shaking from too much life, Merlin's life, or emotion or both, "What ...?"