In Greek mythology, Styx is a deity and a river that forms the boundary between Earth (Gaia) and the Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the centre of the underworld on a great marsh, which sometimes is also called the Styx. According to Herodotus, the river Styx originates near Pheneus. Styx is also 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.
The deities of the Greek pantheon swore all their oaths upon the river Styx because, according to Greek mythology, during the Titanomachy, Styx, the goddess of the river, sided with Zeus. After the war, Zeus declared that every oath must 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. Myths related to such early deities did not survive long enough to be included in historic records, but tantalizing references exist among those that have been discovered.
According to some versions, the river Styx had miraculous powers which could make someone who bathed in the waters invulnerable. According to one tradition, Achilles' mother dipped him in the river during his childhood and he thus acquired invulnerability, with the exception of the heel by which his mother held him. Achilles was struck and killed during the Trojan War by an arrow shot into his heel by Paris. 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. The ferryman Charon is sometimes described as having transported the souls of the newly dead across this river into the Underworld, although in many sources it is instead the Acheron which Charon crosses and which is at the entrance of the Underworld. The further down the Styx people were carried, the longer and/or more severe would be their punishment. Dante, putting Charon and the Acheron at the entrance to Hell, 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.
Most historical accounts, including Pausanias and later Dante's Inferno, associate Charon with the river Acheron. Ancient Greek literary sources – such as Pindar, Aeschylus, Euripides, Plato, and Callimachus – also place Charon on the Acheron. Roman poets, including Propertius, Ovid, and Statius, name the river as the Styx, perhaps following the geography of Virgil's underworld in the Aeneid, where Charon is associated with both rivers.
In ancient times some believed that a coin placed in the mouth of a dead person would pay the toll for the ferry across the river to the entrance of the Underworld. It was said that if someone could not pay the fee, they would never be able to cross the river. The ritual was performed by the relatives of the dead. According to the myth Narcissus is still admiring himself in the Underworld, looking at the waters of the Styx