In Greek mythology, the Styx was the river that separated the world of the living from the Underworld. Its name is associated with hatred or loathing, and its waters carried extraordinary divine power.
The Styx was also a divine personification, the daughter of Oceanus and Tethys in many genealogical traditions. During the war between Zeus and the Titans, Styx and her children supported Zeus. In return, the gods swore their most binding oaths by her waters.
A god who falsely swore by the Styx could suffer a severe punishment, traditionally losing the ability to speak or breathe divine nectar for a period of years. This made the river central not only to death but also to divine law and trust.
Greek descriptions name several Underworld rivers, including the Lethe, associated with forgetfulness, and the Acheron, associated with pain or woe. Later traditions do not always assign the Styx exactly the same boundary function, but it remains the most famous river linked with passage into Hades.