He has been there for millions of years. Reliving his own life, a thousand, thousand different ways.
Every time, he is not allowed to die, or be at peace. Rather, he is brought back here, after his failure. To protect Arthur's destiny, scattered by the greed of Morgause through the Spell of Shattered Sight.
Merlin lays on the floor of the Crystal Cave, sobbing. It is the first time in all the long ages that he has been dropped into this cave after the Legend's end, that he has utterly lost hope.
No matter what he does, Albion is never born.
At least, not yet.
Merlin doesn't know that he was waiting for the right age to come. That Albion is not a single place or person, but a kingdom of magic keepers.
That out of Arthur, and the magic that created him, one day would come the Savior of the Enchanted Forest, the product of true love.
That it is only through Arthur's death that the magic of true love can weave into the fabric of all natural magic and give the Savior her power.
That Merlin's undying love for the other half of the coin is what has created the very magic that Emma would be born from. The power of true love and sacrifice that will generate her life force is the very bond of Albion.
Merlin doesn't know that, for the eons of his suffering, he has been building Albion stone by stone.
All along he has been the champion.
But Albion's need is dire now, because its champion is losing his mind.
Losing his memory and will, at last.
Merlin screams. His own despair has undone him. Without his control or willpower to temper his gifts, he releases one tremendous wave of power, enough to kill him. It shoots from his hands before he can stop it, before he can actualize that he is accidentally killing himself, and possibly forfeiting his eternal loop of Destiny.
The Crystal Cave shatters and the shards of it shrink and come back to Merlin. They are pulled into him, into his bones, like icy little diamonds, sewing him together.
The moment Merlin dies for the first time in all of his Immortality is the moment he is revived.
Even the death throes of immortal magic are so powerful they fold in on themselves. At the very moment Merlin's own magic kills him, it brings him back to life, and so he feels no pain.
His eyes go wide as that power surges through him. He knows through its cold that he has met Death at last, but he also feels himself revived, all in a single moment.
In a twinkling of glass, he is standing in the remnant of the Crystal Cave. It is daytime in the Enchanted Forest, deep in the heart of what was Albion millions of years ago.
Merlin forgets that he is the other side of the coin. His dire need, and magical death, have created an event that has never happened before in the history of the Worlds. All of Destiny has been flipped, like an hour glass.
In the millions of years since Arthur Pendragon died in his true final battle, a portal to another world has gradually opened under Lake Avalon. The portal has drawn away what was once the bottom of the lake, where all the dead were to sink, and make the waters their grave.
The portal opens in the heart of the seas on Earth. The sea then gives up her waves, sending what was the grave of Avalon up to the sea foam, and out to the nearest magic-bearing shoreline.
Far away, in Storybrooke, late at night, the grave dust of Avalon washes to the horizon.
There, under the power of Merlin's Dire Hour Spell, Arthur Pendragon's body sews itself back together. The King of Albion has come back to life.