An irredeemable reversal of the hero's fortune in a tragedy is called.

The term that describes an irredeemable reversal of the hero's fortune in a tragedy is "peripeteia". It refers to the moment in a tragic plot when the protagonist's fortune suddenly changes from good to bad, usually because of a mistake they make or an unexpected turn of events. This change of fortune is irreversible and leads to the tragic conclusion of the play. For example, in the play "Oedipus Rex" by Sophocles, the peripeteia occurs when Oedipus, who has been trying to solve the mystery of his true identity, discovers that he has unwittingly killed his father and married his mother. This revelation marks the irreversible turning point in the plot and ultimately leads to Oedipus's downfall.