Circe advises Odysseus that in order to complete his journey home he must make a detour to the underworld. There, the spirit of the blind prophet Tiresias awaits. He has something very important he wishes to tell Odysseus, something that will be of enormous importance as he proceeds with his epic journey. Against the wishes of his men, Odysseus does as he is bid and descends to the underworld.
Once there, Odysseus is told by Tiresias that Poseidon, god of the sea, is very angry with him for blinding his son Polyphemus the Cyclops. However, Tiresias advises Odysseus that he and his men should be fine, providing that they exercise due caution. Among other things, this means Odysseus cannot harm the cattle of Helios, the sun god. If they ignore his advice, however, then Odysseus will return home a broken man, alone without his shipmates, all of whom will die. The prophet also instructs Odysseus to carry his oar inland until he meets a race of men who know so little about the sea that they think the oar is "a fan to winnow grain." Once he's completed this feat, he needs to make a sacrifice to Poseidon to appease his wrath for what Odysseus did to Polyphemus.