In Greek mythology, Styx is both a goddess and one of the rivers of the Underworld, which she personifies. The eldest of the Oceanids, daughters of the Titan Oceanus and his sister-wife Tethys, she was the first deity to side with Zeus during the Titanomachy, the war against the Titans. As her reward Zeus made the water of the Styx the binding oath of the gods, the most solemn vow a deity could swear, and granted her children a permanent place at his side. An actual Arcadian stream near Nonacris, falling from a high cliff, was identified in antiquity with the river of the dead.
Adapted from Wikipedia, "Styx" · CC BY-SA 4.0
I.Family
Styx was the eldest of the Oceanids, the many daughters of the Titan Oceanus, the great world-encircling river, and his sister-wife, the Titaness Tethys. As a daughter of Oceanus she belonged to the first generation of water deities, alongside her many sisters and her brothers the river gods.
She married the Titan Pallas, and by him became the mother of four personified abstractions who attended Zeus: Zelus (Glory or Emulation), Nike (Victory), Kratos (Strength or Dominion), and Bia (Force or Violence). These children embody the qualities of sovereign power and would stand permanently beside the king of the gods.
II.Role in the Titanomachy
During the Titanomachy, the war between the Olympians and the Titans, Zeus summoned the deathless gods and promised that those who fought at his side would keep their honours and rights, and that any who had held no station under the previous ruler would be granted one. On the advice of her father Oceanus, Styx was the first deity to answer the call, coming to Olympus bringing her children Zelus, Nike, Kratos, and Bia with her.
As a reward for her loyalty, Zeus appointed Styx to be the great oath of the gods and decreed that her children should dwell with him forever. In this way the eldest of the Oceanids came to occupy a position of unique honour among the immortals.
III.The oath of the gods
Swearing by the water of the Styx was described as the greatest and most dread oath for the blessed gods. When the gods wished to make an unbreakable vow, Iris was sent to fetch water from the river in a golden ewer, and the oath was sworn upon it.
A god who swore falsely by the Styx faced a severe punishment. The perjurer was said to lie breathless for a full year, cut off from ambrosia and nectar, and then to be banished from the company and councils of the other gods for nine further years before being allowed to rejoin them.
IV.The river
Like her father Oceanus and his sons the river gods, the goddess Styx was also a river — in her case a river of the Underworld, encircling the realm of the dead. In myth and poetry the Styx is one of the principal waters that the souls of the dead must cross to reach the world below.
The divine river was associated with a real Arcadian stream near the town of Nonacris, described by ancient writers as a small flow of water dropping from a high cliff into a pool. Its waters were credited with strange and dangerous properties, and the site was treated as a tangible point of contact with the river of the gods' oaths.