The River Styx (Greek: Στύξ, Stux, also meaning "hate" and "detestation") (adjectival form: Stygian, pronounced /ˈstɪdʒi.ən/) was a river in Greek mythology which formed the boundary between Earth and the Underworld (often called Hades which is also the name of this domain's ruler). It circles the Underworld nine times. The rivers Styx, Phlegethon, Acheron, and Cocytus all converge at the center of the underworld on a great marsh. The other important rivers of the underworld are Lethe and Eridanos, and Alpheus. The ferryman was called Charon.
The gods respected the Styx and swore binding oaths on it. Zeus swore to give Semele whatever she wanted and was then obliged to follow through, resulting in her death. Helios similarly promised Phaëton whatever he desired, also resulting in his death. Gods that did not follow through on such an oath had to drink from the river, causing them to lose their voices for nine years, then being exiled from the council of gods for nine years after that. 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, but has been described as a feature present in the hell of Christianity as well, notably in The Divine Comedy and also "Paradise Lost". The ferryman Charon is in modern times commonly 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.
In ancient times some believed that placing a coin in the mouth or two coins in the eyes of the deceased, would help pay the toll for the ferry to help cross the Styx river which would lead one to the entrance of the underworld. If some 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 Stix was sometimes used in translations of Classical Greek before the 20th century. By synecdoche, the adjective stygian came to refer to anything dark, dismal, and murky.
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