The River Styx is a sacred river that separates the land of the living from the realm of the dead located in the Underworld. It is sometimes simply referred to as the Styx. Oaths made by this river brings something 'worse than death' to the Oath Bearer if not fulfilled.
Styx was a nymph that was born to Oceanus and Tethys. Due to her efforts in the First Titan War, she gained a very important role in Greek mythology. Her children includes Zelus, Nike, Kratos, and Bia.
The river was a sacred place to both gods and men. It circles the Underworld nine times, eventually converging on a great marsh at the center of the Underworld with the rivers Phlegethon, Acheron, and Cocytus (though there is a fourth river: Lethe).