The Styx is a river in Greek mythology that formed the boundary between Earth and the Underworld. The rivers Styx, Phlegethon, Acheron, and Cocytus all converge at the center of the underworld on a great marsh, which is also sometimes called the Styx.
Hades~ Ruler of the Underworld, god of wealth and the dead. He isn't considered an Olympian, however he is invited to Olympus whenever he wants. His three head dog, Cerberus guards his realm from intruders.
Orestes at Delphi flanked by Athena and Pylades among the Erinyes and priestesses of the oracle, perhaps including Pythia behind the tripod - Paestan red-figured bell krater c. 330 BC - Wikipedia, the free encyclopedia