Στύγα, or the Styx, is a river in Greek mythology that forms the boundary between the Earth and the Underworld. It is often depicted as a dark, murky stream that souls must cross to reach the realm of the dead. The river is associated with the goddess Styx, who personifies the river itself and is one of the primordial deities. In mythology, the ferryman Charon transports souls across the Styx, charging a fee typically paid with an obol, a small coin. The river is also known for its magical properties; drinking from it could grant invulnerability, as seen in the story of Achilles.