The Greek soothsayer who interpreted the cause of Apollo’s plague was Calchas. At the beginning of Homer’s Iliad, the Greek army suffers a devastating pestilence after Agamemnon dishonors Chryses, a priest of Apollo.
Calchas explains that Apollo sent the plague because Agamemnon refused to return Chryses’ daughter, Chryseis, to her father. He says the only remedy is to send her back without ransom and make suitable offerings to the god. Agamemnon agrees, but demands Achilles’ captive Briseis as compensation, deepening the conflict between the two leaders.
That quarrel becomes the central human dispute of the Iliad. Calchas is not merely a general adviser: he is a diviner whose knowledge comes from Apollo and who can interpret divine causes behind earthly events. Tiresias is another famous Greek prophet, but he belongs chiefly to Theban stories and is not the seer who diagnoses this plague.