Cassandra’s name, like that of Mentor, another Homeric character, has become a word in the English dictionary, although in her case the word remains capitalised:
Cassa’ndra, n. Prophet of ill; unregarded prophet. (Concise Oxford Dictionary, 3rd edn. 1934)
Colleagues sometimes ask about this woman, especially those who have seen one of the dramas like Agamemnon or The Trojan Women, in both of which she appears as a vivid, and frightening, character.
In the Iliad, Homer calls her the most beautiful of Priam’s daughters —θυγατρῶν εἶδος ἀρίστην (XIII.365). The observant reader may recall that he has said exactly the same about Laodice (VI.252). But he seems to clinch it in Book XXIV, when Cassandra climbs the tower of the Citadel and is the first of the Trojans to see her father returning from the Greek camp with the body of her brother Hector: she is ἰκέλη χρυσῇ Ἀφροδίτῃ—looking like golden Aphrodite.
And so there were suitors for her: in the Iliad, Othryoneus from Thrace, who arrived without a bride-price but promised to win her by some valiant deed. He was killed by the Greeks. In Virgil (using Homeric sources) Coroebus, a Phrygian, was apparently accepted by Priam as a future son-in-law, but he too was killed, on the night the city was sacked, as he witnessed Cassandra being dragged away from the altar of Athene by her hair, and rushed in to try and prevent it.
Cassandra of course had known this would happen, and she had warned Coroebus to stay away from the war, but he did not believe her.
This gift of true prophecy had been given her by the most illustrious of her suitors—the god Apollo himself: but, as she tells the Chorus in Agamemnon, she broke her promise to accept him as lover, and so was punished by never being believed.
Her role in that play is terrifying: perhaps even more so in a modern production with a more intimate audience. Having arrived in the chariot beside Agamemnon (such was the tact of that sociopath) she sits in motionless silence for a long while after he and Clytemnestra have gone into the palace. Clytemnestra returns and tries to get her to speak, in vain. Then, when the Chorus, and the audience, think she is not going to speak at all, she suddenly starts screaming at the top of her voice, making everyone jump out of their skins. And if that were not terrifying enough, she relates what atrocities she feels have taken place in the house years before, to the astonishment of the Chorus—who nonetheless disbelieve her prediction of what is going to happen now.
And so she dies, along with the king whose slave concubine she has become, but not before she has prophesied the series of murderous revenges that will follow their deaths.
Later Greeks supposed that she was not just the sister but the twin of Helenus, who also had the gift of prophecy: however, people believed what he said. He was the only one of Priam and Hecuba’s sons who survived the war, having earlier survived a wound from Menelaus, whose spear went through his hand. Helenus appears in the Aeneid, reigning now in Epirus, where he receives Aeneas on his voyage to Italy, warns him of the dangers he will meet, and tells precisely the spot where the new city is to be built. (Aeneid III.374ff)