In Greek mythology, the gods swore their most binding oaths by the River Styx.
The Styx was a divine river associated with the boundary between the worlds of the living and the dead. The gods regarded an oath sworn by its waters as absolutely binding. Hesiod’s Theogony describes the goddess Styx as the daughter of Oceanus and Tethys, and as a powerful ally of Zeus during the Titanomachy.
A god who falsely swore by the Styx faced a severe punishment: according to Hesiod, the deity became breathless and senseless for a year, then endured a long period of exclusion from the divine community.
The Styx is often treated as the main river of the Underworld, but classical sources also name Acheron, Cocytus, Phlegethon, and Lethe. These rivers have different associations, so they are not interchangeable.