Styx was the son of Night; he had fought on the side of the Olympians, and for this reason Zeus accorded him the privilege of being guarantor of vows made by the gods. Whenever a god wanted to bind himself by a vow, a ewer of water was drawn from the Styx and taken back to Olympus as a witness to the vow. If the god perjured himself, he was deprived of breath for an entire year, and for nine years he lived away from divine assemblies. The name of Styx was given a stream in Arcadia whose waters were supposed to have harmful properties; it was a strong poison broken up by metals that were thrown into it. This stream was considered to be a resurgence of the underworld river.
In another tradition, Styx was a Greek chthonic underworld goddess, the daughter of Okeanos and Tethys, and mother of Nike. She was the deity of the river Styx beside which the gods swore their oaths. A.G.H.
Grimal, Pierre, Larousse World Mythology, Secaucus, New Jersey, Chartwell Books, 1965, p. 136
Jordan, Michael, Encyclopedia of Gods, New York, Facts On File, Inc. 1993, p. 243