As we all found out in the finale seconds of the last episode, Merlin is immortal. When it gets a closer look, there were hints about it more than once in the show. But most importantly - Merlin doesn’t just live forever. He can not simply die.
And Merlin can not die until Arthur returns.
Let me explain this a little more.
Firstly I’d like to point out that ‘Emrys’, Merlin’s druid name, means immortal. A lot of us knew that already, but let’s get into the really interesting and poignant fact - being immortal doesn’t mean he just lives forever, it means he can’t die at all.
I always wondered why it was always Arthur nearly dying or almost getting killed in the episodes. Merlin had just a few close-calls during the five seasons, but the fact that he was immortal was hinted to us more than once. When? Let me tell you.
The first hint is as early as during the episode Poisoned chalice (1x04), Merlin drinks the poison for Arthur and starts dying. In the end, Gaius makes a magical antidote for him, and he makes Merlin to drink it. But Merlin stops breathing only mere seconds after drinking it. Gaius then listens to his heart but Merlin’s heart has already stopped. Which means his heart wouldn’t make his blood circulate and let the antidote fight the poison inside his veins. Also, I don’t think Gaius would get these things wrong, don’t you think, being a trained physician and all?
No. Because the fact is that Merlin dies in this scene and comes back to life. The antidote has nothing to do with it.
Then on the episode Le morte D’Arthur (1x13) Arthur gets injured by the questing beast and is doomed to die soon. Merlin travels to the Isle of the Blessed and trades his own life for Arthur’s. Merlin goes back to Camelot and waits all night for death to claim him, but he doesn’t die. Instead, it’s his mother that falls fatally ill. The Old Religion leaves him untouched and alive - as if he can not die at all.
Alright. Starts making sense? Then, we get to the Curse of Cornelius Sigan (2x01). Sigan is an immortal spirit, and if he posesses someone, they can’t handle the power and die. Let me quote Gaius from that episode
Gaius: “Merlin, Sigan’s power is far beyond yours. (…)He is immortal and you are not. If you face him, he will destroy you.”
Now when Merlin challenges Sigan, Sigan has posessed the body of Cedric. When Merlin refuses to join Sigan, Sigan leaves Cedric’s body and floats to Merlin and posesses him instead. We see Merlin falling to the ground, convulsing, until his eyes turn black - indicating that Sigan has truly gotten a hold of his body. In this moment, Merlin should have died. There is no way he should have stayed alive after Sigan posessed him, because Cedrid died instantly. Merlin, however, stays alive. He destroys Sigan and survives again miraculously.
Are you starting to believe me now? But we’re not done yet, not nearly.
There is also the time Merlin gets stung by a Serket scorpion in Tears of Uther Pendragon (3x01) He seems to pass out from the poison, but does he, really? What if he dies again? Kilgarrah claims later on, when Merlin wakes up, that he has healed Merlin. About this, we can’t be really certain what happened.
Then there’s the most important incident with the Dorocha in the two-parter Darkest hour (4x01-4x02) The Dorochas are the deadly spirits whose touch kill instantly every mortal. Let me once again quote Gaius from that episode:
ARTHUR: “How do we defeat these creatures?”
GAIUS: “I don’t know, sire. No mortal has ever survived their touch.”
So the spirits always kill the victims. But then we get to the end of the episode where Merlin jumps in front of the Dorocha to protect Arthur, and the Dorocha spirit goes through him, throwing him against a wall. Merlin falls and when Arthur rushes to him and turns him around, Merlin’s eyes are open and he looks as frozen as the other victims before. I had never realized before that Merlin actually dies in this scene. He is dead. Right there, when Arthur turns him around, Merlin is dead. Just like the other victims before him. But he comes back alive some time after, because in the next episode’s beginning, we see Merlin gravely ill but very much alive.
There are several injuries and knock-outs in the episodes between, but they are not life-threatening or serious. Let’s move to the last season.
So, let’s talk about the episode Another’s sorrow (5x05) where Morgana throws Merlin into a tree with magic and casts a spell on him - we see Merlin starting to choke and he seems to pass out.
Then Merlin is brought to Gaius’ care. Gaius says that Merlin has received a ‘heavy blow to the head’, but after a long time, Merlin doesn’t wake up. Let me quote the actual dialogue from the episode:
Gwaine: “He should’ve come around by now. “
Gaius: “It’s unusual certainly.”
Gwaine: “Just a blow to the head, you say?”
Gaius: “There is nothing to suggest anything more. On the outside at least. He’s getting cold…”
Gaius tries to heal Merlin with a spell, but he fails. He tries again, but there is no change. Gaius looks desperate and whispers pleadingly to Merlin that he doesn’t have enough magic to do more powerful spell than this. In other words, it couldn’t have been Gaius’ spell that healed Merlin. But like by pure miracle, Merlin soon opens his eyes and is brought back to the lands of the living again.
Or precisely - he wasn’t healed. He even might have been dead for a moment - that’s why Gaius said he was getting colder (and also to get Gwaine leave). So Merlin died. He came back to life. Again.
Are you starting to see a pattern here? It makes awfully lot of sense, doesn’t it?
When we reach the episode of Kindness of strangers (5x10) Merlin receives an arrow to his side. He soon gets very sick and weak, and later he has lost so much blood that he just collapses to the ground, merely being able to call the dragon to help him before his eyes slid closed. Whether it was Kilgarrah that healed him or Merlin dying again and coming back to life, we’ll never know. My best guess is that Kilgarrah didn’t save Merlin at all during this or back with the Serket poison - he just didn’t tell Merlin that he had, in fact, died, and simply came back to life.
Then finally, we get to the last episode (Diamond of the day, part 2. 5x13) As well all know, Arthur ends up dying to Merlin’s arms. More importantly, we also find out that Morgana as a High Priestess and powerful sorceress can only die by a blade that was forged in the dragon’s breath - so by Excalibur, since Mordred’s sword was buried with him. This is important to know.
In the end, Kilgarrah tells Merlin that one day Arthur will rise again when he is needed. Merlin buries Arthur to the Lake of Avalon. He hesitates on the shoreline for a moment - and I have to admit that for a moment I was absolutely certain that Merlin thought about running himself through with Excalibur. I think he already had some vague idea about his immortality by now - and according to Morgana, Excalibur might be the only weapon in the world that could kill him permanently. But Merlin doesn’t test this theory, instead he throws Excalibur to the lake, and a hand comes up from the water to grab it. This was confirmed to be Arthur’s hand by the writers - and I find it truly adorable that Arthur takes the sword, making sure Merlin doesn’t have the option to try and kill himself.
Then Merlin is left alone. As it seems, he never returns to Camelot. As years go by, his mother and his friends like Gwen, Leon and Percival start to die of old age. Merlin, however, doesn’t. He keeps ageing but never in fact, dies, or at least stays dead.
Merlin’s whole world was shattered when Arthur died. As sad as it sounds, I think it’s safe to assume that Merlin wanted to die at some point after spending hundreds of years alone. He must have tried killing himself. He must have wanted to join Arthur and his loved ones in the afterlife, but couldn’t. I’m sure Merlin realizes at some point that he doesn’t only live forever, but keeps coming back to life even if he dies.
So, in conclusion - Merlin is doomed to stay alive as long as it takes for Arthur to rise. He will age but he will not die, waiting thousands of years if he has to. When Arthur died, their destiny wasn’t fulfilled - the magic hadn’t returned to the land and Albion wasn’t united. And until that happens, Merlin and Arthur won’t be together. But when someday Arthur rises, they meet again upon Earth, and when they have completed their destiny together just like they were supposed to, then Arthur can finally draw Excalibur from its sheath and give Merlin the only weapon that can kill him and make them both be finally together - in death. The two halves will once again be whole. Their destiny will be fulfilled, and this time Arthur can take Merlin with him to Avalon, now that Merlin can finally die for good.
And that, my friends, is in its all sadness, quite a beautiful ending to their story.