In Greek mythology, Zeus used the river Styx as the sacred witness for an unbreakable oath by the gods.
The Styx was a river of the Underworld, and divine oaths sworn by it carried the strongest possible force. Hesiod describes the goddess Styx as the daughter of Oceanus and Tethys, and as an ally of Zeus during the Titanomachy. Her waters therefore held a special authority among the Olympians.
The river is often confused with other Underworld waters. Lethe was linked with forgetfulness, while Acheron was associated with sorrow and the passage of the dead. Styx, by contrast, was especially connected with divine promises and inviolable oaths.
The name Styx comes from a Greek word associated with hatred or abhorrence. In later stories, Achilles was dipped in the Styx by his mother Thetis, although that episode is best known from later tradition rather than Homer’s Iliad.