What connection does Oberht make between the deathless man and the diggers in the vineyard?

Over the course of the story, Natalia comes to believe that her grandfather's soul was taken by the Deathless Man, who collects souls such as those recently released in the digger's ritual. She believes that if she participates in the ritual, she will be able to meet the Deathless Man and learn more about her grandfather's last moments of life.