In Greek mythology, Calchas ("bronze-man"), son of Thestor, was a Argive seer, with a gift for interpreting the flight of birds that he received of Apollo: "as an augur, Calchas had no rival in the camp" (Iliad i, E.V. Rieu translation).
It was Calchas who prophesied that in order to gain a favourable wind to deploy the Greek ships mustered in Aulis on their way to Troy, Agamemnon would need to sacrifice his daughter, Iphigeneia, to appease Artemis, whom Agamemnon had offended; the episode was related at length in the lost Cypria, of the Epic Cycle.
In the Iliad, Calchas tells the Greeks that the captive Chryseis must be returned to her father Chryses in order to get Apollo to stop the plague he has sent as a punishment: this triggered the quarrel of Achilles and Agamemnon, the main theme of the Iliad.
Calchas died of shame at Colophon in Asia Minor shortly after the Trojan War (told in the Cyclic Nostoi and Melampodia): the prophet Mopsus beat him in a contest of soothsaying, although Strabo (6.3.9) placed an oracle of Calchas on Monte Gargano in Magna Graecia. It is also said that Calchas died of laughter when the day that was to be his death day arrived and the prediction didn't seem to materialize.
In medieval and later versions of the myth Calchas is portrayed as a Trojan defector and the father of Chryseis, now called Cressida.