what does the shepherd reveal to Oedipus?

A messenger brings what appear to be glad tidings to Oedipus. His father, King Polybus, has died of old age. There is an air of relief around the court. It would seem that Tiresias's prophecy now cannot be fulfilled; Oedipus has assuredly not killed his father. But that still leaves the matter of the other important part of the prophecy: that Oedipus will marry his mother, Merope, who is still very much alive.
The messenger, sensing Oedipus's uneasiness, tries to be helpful and says that Polybus and Merope are not Oedipus's real parents after all. He then tells the story of how he gave the baby Oedipus, abandoned by a shepherd, to the royal couple. Oedipus is determined to hear the shepherd's side of the story, so he decides to track him down. Jocasta, Oedipus's wife, is terrified of what will be revealed.
And she is right to be terrified. For the shepherd reveals the truth about Oedipus's true parentage. Tiresias's prophecy has indeed come true: Oedipus has killed his father and married his mother. The consequences will be devastating for all concerned.