Figures of Earth by James Branch Cabell

Now some of Poictesme -- but not all they of Poictesme, because the pious deny this portion of the tale, and speak of an ascension -- some narrate that after the appalling eucharist which young Jurgen witnessed upon Upper Morven, the Redeemer of Poictesme rode on a far and troubling journey with Grandfather Death, until the two had passed the sunset, and had come to the dark stream of Lethe.

"Now we must ford these shadowy waters," said Grandfather Death, "in part because your destiny is on the other side, and in part because by the contact of these waters all your memories will be washed away from you. And that is requisite to your destiny."