Styx was the underworld river that the gods used when swearing binding oaths.
In Greek mythology, an oath sworn by the waters of Styx was the most solemn promise a deity could make. Homer calls Styx the dread river of oath, and divine beings who violated such an oath faced severe punishment, including exclusion from the gods’ feasts and loss of divine privileges for a lengthy period.
Hesiod explains this role through Styx’s loyalty during the Titanomachy, the war between Zeus and the Titans. Styx was said to have been the first deity to support Zeus, bringing her children—including Nike, personification of victory—to his side. In return, Zeus made her waters the sacred guarantee of the gods’ promises.
Styx is often confused with the Acheron because later art and popular culture frequently portray Charon ferrying souls across the Styx. Ancient sources sometimes use the Styx as the crossing river, while others assign that role to the Acheron. The oath function, however, is specifically associated with Styx.