River Styx

The River Styx courses through the Lower Planes, frustrating every attempt to map it or predict its course. Getting lost while sailing the Styx isn’t the only danger the river presents. Simply tasting or touching its waters can shatter a creature’s intellect and personality, as well as strip away its memories. Certain fiends are immune to the river’s effects, but most creatures have no defense against it.
Unless immune to the river’s effects, a creature that drinks from the Styx or enters the river is targeted by a Feeblemind spell (save DC 20). A creature must repeat the saving throw whenever it starts its turn in the river, until it fails the save. A feebleminded creature can drink from the Styx and swim in its waters without suffering any additional deleterious effects.
If a creature fails its saving throw and remains under the spell’s effect for 30 consecutive days, the effect becomes permanent (no save) and the creature loses all its memories, becoming a near-mindless shell of its former self. At that point, nothing short of a wish spell or divine intervention can undo the effect.
Water taken from the River Styx loses its potency after 24 hours, becoming a harmless, foul-tasting liquid. However, arcanaloths, night hags, and other fell creatures might know rituals that can prolong the water’s potency, at your discretion.