Short answers, pulled from the story.
Cassandra was the daughter of King Priam and a princess of Troy who received the gift of prophecy from Apollo. She became known for speaking true prophecies that no one believed because Apollo cursed her after she rejected his advances.
Cassandra tried to stop the Trojans from entering the wooden horse but they ignored her warnings and dragged her away. Ajax the Lesser later seized her near the temple of Athena and took her as a prize when the Greeks sacked the city.
Agamemnon brought Cassandra back to Mycenae where Queen Clytemnestra murdered both Agamemnon and Cassandra together. Aegisthus and Clytemnestra killed them in the palace courtyard along with their twin sons Teledamus and Pelops.
The term Cassandra complex describes a person whose accurate forecasts are dismissed or disbelieved by others. This modern usage derives directly from the mythological story where Cassandra spoke truth yet remained unheard until it was too late.
Homer described her as the fairest daughter of King Priam in the Iliad while Virgil mentioned her prophesying without showing her on stage. Aeschylus, Euripides, and Seneca the Younger each added unique details about her tragic fate and powers in their respective works.