The Iliad, Book II, the Rage of Achilles

Greek king Agamemnon had taken as a concubine the daughter of a priest of Apollo. His unwholesome act was an affront to the god, and Apollo set a plague loose on the Greeks. After nine days their situation was becoming irrecoverable. The Greek champion Achilles felt that he had to intervene. It was time Achilles said to consult a holy man and make sacrifices to the angry god Apollo.