Cassandra is a daughter of Priam and Hecuba. Although Homer mentions nothing of this, most authors write that she was a seer, whom no one would believe.
She was very beautiful, and Apollo fell madly in love with her. At first, she spurned his advances. To win her, he promised her the gift of prophecy. Still, Cassandra remained a virgin. Apollo could not take back his gift (a gift from a god cannot be taken back), and so he gave her a curse instead. From that moment on, no one believed her prophecies.
After the judging the contest for the golden apple, Paris was preparing to sail to Sparta. Cassandra warned Paris, as well as her father Priam to not let him go, but neither would listen. At the end of the war, she warned the Trojans to not bring the horse inside their walls. Of course, they once again did not listen.
As told in Troiades, when Troy was being sacked by the Greeks, Cassandra took refuge in the temple of Athena. At Athena's altar, Ajax of Locris raped Cassandra. When the spoils of war were divided, Agamemnon claimed Cassandra as his slave. She gave birth to two sons by him on the voyage home. In Agamemnon, she warns Agamemnon of his and his family's fate. Once again, her prophecy is ignored. Along with Agamemnon and her two children, Cassandra is murdered by Clytemnestra and Aegisthus.