Achilles goes to Calchas to discover why the men and beasts are dying. What does Calchas ask in return and what does he tell the men?

In Book I, Achilles calls the Greeks to assembly, the idea put into his head by the goddess Hera, who sides with the Achaeans against Troy. Achilles asks for some prophet or seer to tell them what has caused the plague and what must be done to end it. Calchas, a great prophet, says that he knows the answer, but he makes Achilles vow to protect him once he has revealed it. Achilles vows, and Calchas tells them that the plague has been sent by Apollo in punishment for Agamemnon's treatment of Chryses. To atone for the sin, the Achaeans must give Chryseis back without accepting any ransom and in addition they must give a hundred sacred bulls to Chryses for sacrifice.