In the war against the Titans, Styx was the first to go to Olympus when Zeus called for all those who would fight on his side. He guaranteed her immortality and the rule over the river that bears her name, established her to take the oaths of the gods, and told her that her children would always have a home in his household.
If a god gave their oath upon her river and failed to keep their word, Zeus forced them to drink from its waters. The water was said to be so foul that the god would lose their voice for nine years (another legend says the god would lose their immortality). It was also said to freeze the blood of any mortals who drank it. According to other stories, the Styx had miraculous powers and could turn a human into an immortal. Achilles was dipped in it as a child, which gave him his invulnerability- the exception was his heel, which his mother held him by to dip him into it.
The river itself separates the world of the living from the Underworld, along with four other rivers; Acheron, the river of woe; Cocytus, the river of lamentation; Phlegethon, the river of fire; and Lethe, the river of forgetfulness. The river Styx winds nine times around the Underworld.
click for image one.
click for image two.
click for image three.