What does Teiresias tell Odysseus will happen before he returns to Ithaca?
He tells him something after they make a sacrifice.
In the eleventh book of Homer's Odyssey, Odysseus conjures up the spirits of the dead. One of the first spirits he encounters is that of the famous Theban prophet Teiresias, the same prophet who would appears as a character in Sophocles' Oedipus the King and Sophocles' Antigone.
The spirit of Teiresias tells Odysseus about what will happen before he returns to Ithaca and what he must do after he returns to Ithaca. Regarding what will happen to Odysseus before he returns to Ithaca, Teiresias warns Odysseus about the wrath of Poseidon (because Odysseus blinded his son Polyphemus). The prophet also warns Odysseus to avoid the cattle of the sun god, Helios. If Odysseus and his men do not avoid harming these cattle, then Teiresias predicts that Odysseus may manage to make it home, but that all of his men and ships will be destroyed. Additionally, Teiresias adds that if Odysseus manages to reach Ithaca, he will
come unlooked-for to your home, in sore distress, losing all comrades, in another’s vessel, to find great trouble in your house, insolent men who destroy your goods, who court your wife and offer gifts of courtship. (A.S. Kline translation).
For the subject of Odysseus and his men's encounter with the cattle of Helios, please read Odyssey 12.
The dead prophet Teiresias tells Odysseus that he must sail to the Thrinacian island and do certain things before he can return home.
After Odysseus and his men sail from the island of the Cyclopes in Book X, some of the shipmates of Odysseus believe that Aeolus has secretly given Odysseus a fortune in gold and silver, and they tear open the bag. Unfortunately, when the winds are released, they cause a storm that pushes the ships back to Aeolia where Aeolus refuses to help them again. So, the Achaeans row to the land of the Laestrygonians where they meet more misfortune because some of the crew are eaten. Desperately, Odysseus and his remaining men try to flee, but the Laestrygonians hurl huge boulders at the ships, sinking all the ships but the one belonging to Odysseus.
Then, Odysseus and his men travel to Aeaea, where the beautiful witch-goddess Circe turns some of the men into pigs. But Hermes assists Odysseus and the men are retrieved. However, Odysseus is seduced by Circe and becomes her lover for a year. Finally, Odysseus pleads with her to be allowed to return home. She agrees, but instructs Odysseus that he must first go to the halls of Hades and Persephone and there consult with Teiresias of Thebes.
So, Odysseus must inform his men that they must first go to Hades before heading home. In Book XI Odysseus then goes to the underworld where he speaks with the prophet Teiresias, who informs him that Poseidon is punishing him for having blinded his son Polyphemus (the Cyclops). Also, Teiresias says that when Odysseus brings his "stanch ship" to the Thrinacian island and finds the pasturing "kine and sturdy sheep of the Sun," he must leave them unharmed. He predicts that Odysseus then will be able to return home. Teiresias adds that Odysseus will find that he will have to seize his home from the wretched suitors who have been there for a long time; then, it will be necessary for him to make another trip in order to appease Poseidon by offering him a ram, a bull, and a boar. Finally, he can turn homeward, Teiresias says, and then
"...offer sacred hecatombs to the immortal gods who hold the open sky, all in the order due. Upon yourself death from the sea shall very gently come and cut you off bowed down with hale old age. Round you shall be a prosperous people. I speak what shall not fail."
After visiting with the spirit of his dead mother, Odysseus talks with other shades, then hurries to his ship.