The Styx is a river in Greek mythology that formed the boundary between Earth and the Underworld (often called Hades which is also the name of this domain's ruler). The rivers Styx, Phlegethon, Acheron, and Cocytus all converge at the center of the underworld on a great marsh, which is also sometimes called the Styx.
According to Herodotus the river Styx originates near Feneos.
The gods were bound by the Styx and swore oaths on it. The reason for this is during 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 realized to his horror that her request would lead to her death. Helios similarly promised his son Phaëton whatever he desired, also resulting in the boy's death. According to some versions, Styx had miraculous powers and could make someone invulnerable. According to one tradition, Achilles was dipped in it in his childhood, acquiring invulnerability, with exception of his heel, by which his mother held him. This is the source of the expression Achilles' heel, a metaphor for a vulnerable spot.
Styx was primarily a feature in the afterworld of Greek mythology, and similar to the Christian area of Hell in texts such as The Divine Comedy and Paradise Lost. The ferryman Charon is believed to have transported the souls of the newly dead across this river into the underworld, though in the original Greek and Roman sources, as well as in Dante, it was the river Acheron that Charon plied. Dante put Phlegyas 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 some believed that placing a coin in the mouth of the deceased would help pay the toll for the ferry to help cross the Acheron River which would lead one to the entrance of the underworld. If someone could not pay the fee it was said that they would never be able to cross the river. This ritual was performed by the relatives.
The variant spelling Styx was sometimes used in translations of Classical Greek before the 20th century. By metonymy, the adjective stygian came to refer to anything dark, dismal, and murky.