In Greek mythology, Styx (//; Ancient Greek: Στύξ [stýkʰs]) is a deity and a river that forms the boundary between Earth and the Underworld (the domain often called Hades, which additionally is the name of its ruler). The rivers Styx, Phlegethon, Acheron, Lethe, and Cocytus all converge at the centre of the underworld on a great marsh, which at times is additionally called the Styx. According to Herodotus, the river Styx originates near Feneos. Styx is additionally a goddess with prehistoric roots in Greek mythology as a daughter of Tethys, after whom the river is named and because of whom it had miraculous powers.
Significance of the River Styx
The deities were bound by the Styx and swore oaths upon Styx. According to classical myths, the reason related for this is that throughout the Titan war, Styx, the goddess of the river Styx, sided with Zeus. After the war, Zeus promised every oath be sworn upon her. Zeus swore to give Semele whatever she wanted and was then obliged to follow through when he realised to his horror that her request would lead to her death. Helios similarly promised his son Phaëton whatever he desired, additionally resulting in the boy's death. Myths related to such early deities didn't survive long enough to be included in historic records, but tantalising references exist among those that have been discovered.
According to a few versions, Styx had miraculous powers and could make someone invulnerable. According to one tradition, Achilles was dipped in the waters of the river by his mother throughout his childhood, acquiring invulnerability, with exception of his heel, by which his mother held him. The only spot where Achilles was vulnerable was his heel, where he was struck and killed by Paris' arrow in the Trojan War. This is the source of the expression Achilles' heel, a metaphor for a vulnerable spot.
Styx was primarily a feature in the afterworld of classical Greek mythology, similar to the Christian area of Hell in texts such as The Divine Comedy and Paradise Lost. The ferryman Charon often is described in contemporary literature as having transported the souls of the newly dead across this river into the underworld, although in the original Greek and Roman sources, as well as in Dante, it was the river Acheron that Charon plied. Dante put Phlegyas as ferryman over the Styx and made it the fifth circle of Hell, where the wrathful and sullen are punished by being drowned in the muddy waters for eternity, with the wrathful fighting each other. In ancient times a few believed that placing a coin (Charon's obol) in the mouth of the deceased would pay the toll for the ferry to cross the Acheron River, which would lead one to the entrance of the underworld. If someone couldn't pay the fee it was said that they would never be able to cross the river. This ritual was performed by the relatives.
Styx was the name of the daughter of Tethys and Oceanus, the goddess of the River Styx. In classical myths, her husband was Pallas and she gave birth to Zelus, Nike, Kratos, and Bia (and at times Eos). In those myths, Styx supported Zeus in the Titanomachy, where she was said to be the first to rush to his aid. For this reason, her name was given the honour of being a binding oath for the deities. Knowledge of whether this was the original reason for the tradition didn't survive into historical records following the religious transition that led to the pantheon of the classical era.
As of 2 July 2013, Styx officially became the name of one of Pluto's moons. The additional moons (Charon, Nix, Hydra, and Kerberos) additionally have names from Greco-Roman mythology related to the underworld.