Calchas was the professional seer or soothsayer in the Achaean army. He appears in Book 1, where he points the finger at Agamemnon as the cause of the plague (once Achilles has agreed to protect him). In Book 2 he reveals that the war will last for ten years. He's also chosen by Poseidon in Book 13 for his disguise - but the backs of his knees give him away as a god!