Chryseis was the daughter of Chryses, a priest of Apollo. When the Greeks captured the city of Thebe, a Trojan ally, Chryseis was taken prisoner and Agamemnon claimed her as his prize. Agamemnon was infatuated with her, and refused to except Chryses' ransom. As a result, Apollo sent a plague upon the Greeks.
The other Greeks pressured Agamemnon to give up Chryseis, and he finally relented. As recompense, Agamemnon took Briseis, a concubine of Achilles. With his pride now injured, Achilles refused to fight.