There is a reason that he does not tell Arthur to seek immorality, to live alongside Merlin forever.
“Where are you, Merlin?” says the creature that used to be his king.
He has to bite his tongue until it bleeds to stop the impulse of answering to the thing’s (Arthur’s) voice, as he always does. Instead he keeps his fists balled up by his sides, glued to the walls of the ruined castle, feels the sweat trailing down his skin, the heart that he has ripped in half.
This wasn’t supposed to happen.
A shadow moves, and Merlin knows that it is him, stalking the halls for the only thing that makes him feel alive anymore. He cannot feel anything anymore, no remorse, no laughter, no joy. He kills because he can. He feeds because he can. His eyes are hollow and cold, no longer the blue orbs that soften around his subjects, no longer the Arthur he knows. He cares for nothing but having Merlin now.
Laughter echoes through the corridor, making Merlin tense in fear. It is hollow and filled with hunger, a mere imitation of his lover’s former joy. It makes Merlin want to tear his own ears out and scream out, ‘I’m sorry, I’m sorry, please bring him back, please bring him back!’
But he hears the thing that has replaced his king, chuckling, “You cannot hide from me, Merlin. I’ll always find you now.”
Merlin closes his eyes and hates himself even more. This is my fault. I shouldn’t have brought it up. I shouldn’t have asked him. I should have let it be. I should have listened to Kilgharrah and let everything be. But he hadn’t. And now his other half had become a monster that he did not know.
Immorality is a curse.
That is how Merlin comes to see it in the first few decades after Arthur’s death, after Camelot and in the first bitter years he spends waiting for his king to be reborn for the first time. It is the fate of Emrys, of magic reborn. He must live and witness all of time, intervene when destiny dictates. It is his nature, the magic has whispered to him many times.
He is not completely alone. He has the spirits of Kilgharrah and even Nimueh to keep him company when he goes into meditation. Often, Merlin questions if they are real, or if they are voices that he has conjured up to keep his mind from slipping further into madness.
“What would happen if I asked someone to become immortal for me?” Merlin asks them one day, after the third time that Arthur is reborn, after he watched Arthur die in the second reincarnation, Arthur asking Merlin why they had to be separated in death again.
Nimueh snorts, muttering that it is a stupid idea while Kilgharrah growls at him.
“Young warlock, it is not your place to change a mortal’s nature. The price of eternal life is too high for a mortal to pay. There are always fatal consequences, imbalance the natural order of things. As Emrys, it is your job to protect the nature of magic, of life. It is best not to ask such things again.”
But he asks anyways, on the fifth life, when Arthur is bleeding in a battlefield and Merlin cannot stand watching his king die again and again. His magic lashes out, cradling his lover gently in golden hues. Arthur looks up at Merlin with such (undeserved) trust and Merlin asks the (damned) question, “Do you want to live with me forever?”
He should have not have asked. He should have made his king refuse.
But Arthur, sweet, trusting Arthur, says yes.
The magic changes Arthur. But what awakes to greet Merlin, is not Arthur anymore.
It’s caught him, the creature that wears Arthur’s body and smiles in the same way (but his teeth are smeared with the blood of the others he’s eaten). But when he looks at Merlin, his eyes soften (no, no, don’t do that, stop looking like him) and he holds Merlin as if he is the most precious treasure in the world.
“There you are, Merlin,” it whispers in his ear. “I knew you’d surrender in our game eventually.”
He clings to the undead thing, feels it’s cold hands grasping at his hair, and begins to sob quietly.
I’m sorry, he thinks. I’m so sorry for doing this to you, for asking this of you. I’m so sorry that I love you so selfishly. I’m sorry.
“...The next life won’t be like this,” Merlin whispers.
His eyes glow gold, and before the creature notices the change, flames have escaped his fingertips and he holds the creature tightly, prevents it from escaping the heat forming around their bodies.
They burn together.
Merlin survives (as always) and takes a few days to recuperate.
The creature does not.
In their next life, Arthur remembers nothing. (The creature hadn’t truly been him after all, Merlin thinks in relief, or perhaps it is a desperate hope.) But he still has that ridiculously noble notion that he should become immortal as well, so that they can be together forever.
Merlin doesn’t tell him (can’t tell him) about the awful mistake he made. He doesn’t think he would be able to bear the look of horror on his king’s face.
So he smiles and tells Arthur he’s fine.
(And when he remembers the bitter taste of the creature when he kissed him one last time amidst the burning flames, he realizes that as long as Arthur is human and alive, that he really is.)
“I love you as you are now, for always,” Merlin will say when Arthur asks for immorality.
The words work to placate him, if only for a few more centuries.