Nuisance Hazard (Levels 11–16)
The River Styx courses through the Lower Planes. 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.
Unless immune to the river’s effects, a creature that drinks from the Styx, enters the river, or starts its turn in the river makes a DC 20 Intelligence saving throw. On a failed save, the creature takes 19 (3d12) Psychic damage and can’t cast spells or take the Magic action for 30 days. An affected creature can drink from the Styx and swim in its waters without suffering any additional effects.
The effect can be ended only by the Greater Restoration, Heal, or Wish spell. If the effect isn’t ended after 30 days, the effect becomes permanent, and the creature loses all its memories. 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. Arcanaloths, night hags, and other fell creatures might know rituals that can prolong the water’s potency, at your discretion.