In Greek mythology, Kalchas Thestorides, or Calchas ("brazen") for short, was a powerful prophet.
Through prophesy, he told the Greeks several conditions needed to be met in order to achieve victory in the Trojan War, which he had already predicted would take ten years. The first condition was that Achilles (who had been hidden away by his mother, Thetis, by enchantment) was necessary for victory. The next condition was that Chryseis was to return her father Chryses in order to get Apollo to stop sending arrows to them. That last was that in order to gain a favourable wind to deploy the Greek ships, Agamemnon would need to sacrifice his daughter, Iphigeneia, to appease Artemis, whom Agamemnon had offended.
Calchas died of shame after the prophet Mopsus beat him in a contest of soothsaying.