In book 11 of the Odyssey, Odysseus travels to the underworld to gain some information. There are some things he needs to know about his journey and about what has happened at home, and he figures the dead can tell him better than anyone else.
Odysseus learns from Tiresias that he has majorly irritated the god Poseidon by killing the Cyclops, who is Poseidon's son. Tiresias also warns Odysseus not to kill the Cattle of the Sun. If his men harm them, they will not make it home. Odysseus will make it home, but only through great hardship, and he will have to appease Poseidon with sacrifices before he will find true peace.
Odysseus also speaks with his mother and discovers that his wife, Penelope, is waiting for him at home, grieving. Odysseus's estate is secure for the moment, but his father is poor. His mother died of grief for her son as she waited for him to return
As Odysseus talks to Agamemnon, he learns how the latter died, murdered by his wife's lover. Agamemnon advises Odysseus not to trust his own wife too greatly and to go home in secret to assess the situation.
Finally, Achilles tells Odysseus that life in the underworld is no fun. Achilles would much rather return to earthly life as a slave than be a king where he is.