All That You've Given Me
It had been more than a thousand years since his death, and, frankly, Arthur was tired of counting, when he was summoned by the horn of Cathbad. There he met a young woman dressed in weird clothes, who said that Merlin had found out that the prophecy was incorrectly interpreted a few centuries ago, the world had more or less ended a century after that and he had managed to kill himself a year ago. She tells him that there is a way to change the past and prevent all that tragedy from happening if they use a spell she just finished, that would send his mind back into his younger body. The catch is that he will feel unwell for at least a few days and he will also have her mind in his head until he visits the crystal cave at Samhain where the magic will give her a separate body. He agrees and the spell is performed.
Merlin drags the prince's unconscious body from the lake. As Arthur isn't waking up, he carries him all the way to Camelot. When Arthur finally wakes up that night, he apologizes to Merlin and promises to do better in the future. Then he admits that he's from the future, that he knows about Merlin's and Morgana's magic and the prophecy and the dragon and they tell Morgana, and she doesn't turn evil, and somehow they fulfill the prophecy without anyone else turning evil or dying, and then they all live happily ever after.