In Greek mythology, Calchas son of Thestor was the chief bird-diviner of the Achaean host at Troy—gifted by Apollo to know past, present, and future. He named the plague's cause at the opening of the Iliad, read the sparrow omen of ten years' war, and in later epic demanded Iphigenia's sacrifice at Aulis; he died of grief at Colophon when Mopsus outprophesied him.
Adapted from Wikipedia, "Calchas" · CC BY-SA 4.0
I.Seer of the Achaean camp
Calchas is son of Thestor (and, in some lists, Polymele), grandson of the seer Idmon, and far the best of bird-diviners in Homer's camp. In Iliad 1 he declares—under Achilles' protection—that Apollo's plague will lift only when Agamemnon returns Chryseis; Agamemnon rages at the "prophet of evil," then seizes Briseis and sparks the quarrel with Achilles. Poseidon later rouses the Greeks in Calchas' likeness. The Cypria and later tradition also credit him with Iphigenia's sacrifice at Aulis and with the prophecy that Troy falls in the tenth year.
Quintus of Smyrna's Posthomerica expands his postwar counsel: Philoctetes must be fetched, strategy (the Horse) will take the city, Aeneas will survive. Medieval romance sometimes makes him a Trojan defector and father of Cressida; he appears in Shakespeare's Troilus and Cressida.
II.Death at Colophon
The Cyclic Nostoi and Melampodia have him die of shame when the younger seer Mopsus bests him in a contest of soothsaying near Colophon; Apollodorus buries him at Notium after he and Amphilochus leave the ships by land. A comic variant has him die laughing when a rival's prophecy about wine from his own vines seems false. Strabo also places an oracle of Calchas on Monte Gargano in Magna Graecia.
The atlas id calchas-thestor marks the Thestorid Achaean seer against any later literary doubles.