One of the greatest seers, who helped the Greeks at the Trojan war by telling them the wishes and moods of the gods.It was he who told Agamemnon that he must sacrifice his daughter Iphigenia in order for the Greek fleet to be able to sail off, it was he who saw that the war would take ten years and it was also Calchas who understood that the Greeks suffered plague because Apollo was upset about Agamemnon taking Briseis as his mistress. Calchas had also seen that Troy would not be defeated if Achilles did not fight with the Greeks.