When Merlin's life finally faded away, he was content to leave the world. It no longer needed him, nor his king, and he could rest at last. But he couldn't help his concern for the magical world's well-being; He had devoted his life to the spread, cultivation and preservation of magic, and he hoped the people of the future would utilize their talents for good, unlike Morgan.
Deep down, however, Merlin knew the world would only need him should it be in the greatest of peril, and with that, he settled his consciousness down in hopes of finally reuniting with Arthur again.
But, any prospects of an afterlife vanished immediately after as instead of sleeping, he abruptly awoke, in the body of a small child and a room he could not recognize for the life of him.
Well, that plan was botched.