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This article is about the mythological river. For the musical group, see Styx (band). For other rivers named "Styx", see Styx River (disambiguation). For other uses of the word "Styx", see Styx (disambiguation).
In Greek mythology, the "River Styx" (Greek: Στύξ also meaning hate, detest) was a river 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, a real river that runs in Italy partially underground and undersea. Married couples would throw rings into the river to appease the gods of marriage, Hades and Persephone, as it was believed to be one of the few mortal portals to the underworld. In other versions, Phlegyas guarded Phlegethon, one of the other main rivers of the underworld. Sometimes the ferryman was called Charon (also spelt Kharon in older texts).

The gods respected the Styx and swore binding oaths by 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 one year, 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 immortal/invulnerable. Achilles may have been dipped in it in his childhood, acquiring invulnerability, with exception of his heel, which was held by his mother in order to submerge him. His exposed heel thus became known as Achilles' heel, a metaphor for a weak 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.

The variant spelling Stix was sometimes used in translations of Classical Greek before the 20th century. The adjective Stygian means "of, or relating to, the River Styx", and may also refer to anything that is dark and dismal.