What significant revelation does the old servant ultimately disclose?

The old servant’s significant revelation regarding the prophecy that dictated the child's fate is crucial to the development of the story and the overarching themes of fate and free will in "Oedipus Rex." This disclosure underscores the unavoidable nature of the prophecy which foretold that Oedipus would kill his father and marry his mother. The revelation serves as a pivotal moment in the narrative, linking the past actions of both Oedipus and the old servant with the tragic events that unfold.

This moment highlights the power of fate over the characters’ lives—despite their attempts to escape or alter their destinies, the prophecy ultimately dictates the course of their actions and the tragic outcomes. The old servant's acknowledgment of the prophecy not only enhances the dramatic irony already present in the play but also reinforces the idea that the characters are caught in a web of fate from which they cannot escape.