Cassandra was a priestess of Apollo in Troy before the Trojan War. She was very beautiful, and Apollo saw her and fell in love with her.  He offered her the gift of prophecy (being able to see what was going to happen in the future) if she would kiss him. She agreed, and he gave her the gift, but when he went to kiss her she spit in his mouth. Apollo was very angry. He could not take away her gift, but he changed it so that she would always know what was going to happen, but nobody would ever believe her when she told them.
Sure enough, Cassandra told all the people in Troy to watch out for the Trojan Horse, but nobody paid any attention. After the Trojans lost the war, the Greeks took Cassandra prisoner and gave her to Agamemnon as a slave. He took her home to Mycenae, where she warned him that Clytemnestra was going to kill him, but again no one believed her. After killing Agamemnon, Clytemnestra killed Cassandra too.