Acheron

The Acheron is a river in Epirus and is one of the main rivers of the Underworld. Souls are ferried across it by Charon and forms the outer border of Hades' kingdom. Those who have not received the proper burial items (and are thus unable to afford Charon's fare) are destined to wander on one side of the Acheron's banks. The other side is guarded by Cerberus.