Calchas was a seer accompanying the Greeks during the Trojan War. He was so reliable that Agamemnon came personally to his home in Megara to entreat him to join their expedition. He seems to have been under the guidance of the goddess Artemis.
He predicted that the Greeks would not win, unless Achilles accompanied them, although he was probably too young to fight at the time the war began.
At Aulis, the Greeks waited day after day for a favorable wind. The Greeks consulted Calchas, who said that Agamemnon must sacrifice his first born daughter Iphigeneia to appease Artemis. Iphigeneia was told that she was going to marry Achilles, brought to the temple, and either sacrificed or saved by Artemis, as told by later tales.
Euripides. Iphigeneia e en Aulide.