the River Styx

From Wikipedia...

"The River Styx (Greek: Στύξ, Stux, also meaning "hate" and "detestation") was a river in Greek mythology 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. The ferryman was called Charon (also spelled Kharon in older texts).

"Styx was also the name of the daughter of Oceanus and Tethys. With Pallas, she was the mother of Zelus, Nike, Cratos and Bia (and sometimes Eos). Styx supported Zeus in the Titanomachy where she was the first to rush to his aid. For this reason her name was given the honor of being a binding oath for the gods.

"Styx is specifically a Naiad, or fresh water nymph."