Styx - River in Greek mythology that formed the boundary between Earth and the Underworld

"In Greek mythology, Styx is a deity and a river that forms the boundary between Earth and the Underworld. The rivers Acheron, Cocytus, Lethe, Phlegethon, and Styx all converge at the center 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. - Wikipedia"