“…as evening fell they camped beside the River of Unheeding, whose water no jar can hold; and all were bound to drink a measure of it.”
The hint
Before the souls return they drink from the river on the plain, and forget. The key is the river's own name. Five letters.
The cipher
ZFEPZTSG
— Plato · Republic, X, the Myth of Er · c. 375 BCE
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