Location from Percy Jackson by Rick Riordan
The river of hate that flows through the Underworld — bathe in it and gain invulnerability, except for one mortal point. The catch is that the bathing will probably kill you.
The River Styx offers Achilles' Curse — total invulnerability with one mortal weak point — to anyone brave or desperate enough to bathe in it. The water is dark, cold, and saturated with the broken dreams and bitter resentments of every soul that has ever crossed into death. Percy submerged himself to prepare for the Battle of Manhattan, and the experience nearly dissolved him — he survived by anchoring himself to his strongest human connection, his memory of Annabeth. Luke also bathed in it, choosing the small of his back as his mortal point. The river whispers with the voices of the dead, and its current pulls toward oblivion.
A dark, polluted river flowing with the broken dreams and resentments of the dead. Its water is cold, dark, and radiates despair. The banks are grey and lifeless.
Also known as: the River Styx, the Styx