Styx is both a deity and a river that forms the boundary between Earth and the Underworld. The rivers Styx, Phlegethon, Acheron, Lethe, and Cocytus all converge at the center of the underworld on a great marsh, which sometimes is also called the Styx.
In some 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 during 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.
The river Styx later became identified as the river of Hell itself, and became the most famous one. 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, it was the river Acheron that Charon plied.
However, it means not that Charon is denied access from the Styx, for he is seen ferrying the dead across the river. The demigod Phlegyas is placed as the guardian 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.