Calchas was the Greek seer who revealed why Apollo had sent a plague against the Greek army. At the opening of the Iliad, the Achaeans suffer illness after Agamemnon refuses to return Chryseis, the daughter of Apollo's priest, Chryses.
Calchas explains that the plague will end only when Chryseis is returned to her father and proper restitution is made. His prophecy identifies the religious cause of the crisis: Agamemnon has dishonored Chryses, and Apollo is punishing the entire army. Agamemnon eventually agrees to give up Chryseis, but demands Achilles' captive Briseis in compensation.
That demand triggers the central quarrel of the Iliad. Achilles withdraws from battle, allowing the Trojans to gain ground and setting the poem's events in motion. Calchas is not simply a plague specialist: Homer describes him as an exceptionally powerful interpreter of divine will, with knowledge of the past, present, and future. In later Trojan War traditions, he also predicts events connected with the Greek expedition and Troy's fall.