Styx is a deity and a river that forms the boundary between Earth and the Underworld. The rivers Acheron, Cocytus, Lethe and Phlegethon all converge at the center of the underworld on a great marsh, which is also called the Styx.

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.

Charon often transports the souls of the newly dead across this river into the underworld.