This river was so respected by the gods of greek mythology that they would take life binding oaths just by mentioning its name, as in the story of Bacchus-Ariadne, where Jove "confirms it with the irrevocable oath, attesting the river Styx." If a god gave his oath upon the river Styx and failed to keep his word, Zeus forced that god to drink from the river itself. The water is said to be so foul that the god would lose his/her voice for nine years.
There are five rivers that separate Hades from the world of the living, they are: Acheron - the river of woe; Cocytus - the river of lamentation; Phlegethon - the river of fire; Lethe - the river of forgetfulness; Styx - the river of hate.
It is thought that Charon, the old ferry man who ferries the dead onto the underworld, crosses the river Styx where the dragon-tailed dog Cerberus guards allowing all souls to enter but none to leave. This is a misconception because Charon crosses the river Acheron where Cerberus stands his eternal guard. Also, Charon only takes the souls across that are buried properly with a coin (called an obol) that was placed in their mouths upon burial.