The journey to the underworld of Hades to speak to the dead Theban prophet Teiresias is a command given to Odysseus by the enchantress Circe. Circe, who was initally hostile to Odysseus and his men (she changed them into pigs on their first meeting) has become a good host. She has been Odysseus' lover, and hosted him and his crew on her island for a year. But the crew and Odysseus now desire to go home. When Odysseus tells her that it is time to leave, she replies:
Zeus-sprung son of Laertes, Odysseus of many devices, do not stay any longer against your will in my palace. Yet you first must accomplish a different journey and go down into the palace of Hades and Persephone, honored and dreaded, seeking prophetic advice of the Theban Teiresias' spirit, that the blind prophet in whom is a mind still fresh in its vigor;
The idea of a journey to the underworld terrifies Odysseus, but he eventually realizes that he must go. Circe, a goddess, knows that this is a necessary step in his pathway home to Ithaca. When Odysseus gets to the realm of Hades, he talks with Teiresias. The prophet gives him important advice about the perils he is about to undergo, and what awaits him on his home island of Ithaca. Without this advice Odysseus would not have ever reached home.
Text Source: Homer. The Odyssey. Rodney Merrill, trans. Ann Arbor: U of Michigan Press, 2006.