In Greek mythology, Styx (; ) is a deity and a river that forms the boundary between Earth and the Underworld (the domain often called Hades, which also is the name of its ruler). 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. According to Herodotus, the river Styx originates near Feneos. 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.