Answer:
Cassandra was a Trojan, daughter of King Priam and blessed by the god Apollo with the gift of prophecy, but with the curse of never being believed. Captured by the Greeks after the the fall of Troy, she was taken back to Greece as a trophy by, I think, Agamemnon, and prophesied that he would die an agonising death. He had sacrificed his daughter Iphigenia to get a favourable wind from the Gods to enable the Greek fleet to sail for Troy (initially to recapture the wife of his brother Menelaus - Helen - who had been abducted by the Trojan, Paris) 11 years earlier, and his wife Clytemnestra had meantime taken a lover, Aegisthus, and the two of them murdered him in his bath on his return. He had not believed Cassandra, however, who went mad.
Figuratively, a 'Cassandra' is someone who tells bad news (and is perhaps not believed).
I think the name is the same in Spanish.