Cassandra

Cassandra, in Greek mythology, was one of the daughters of King Priam of Troy (ancient city) and his wife Hecuba. The god Apollo granted her the gift of prophecy in return for a promise that she would lie with him. When she went back on her promise, he could not withdraw his gift, but in kissing her, spat into her mouth, with the effect that no-one believed her prophecies, though these were invariably true. She survived the sack of Troy. Some versions of the legend say that Ajax the Lesser raped her in the temple of Athena, others that Odysseus falsely accused him of the rape to please Agamemnon, who wanted her for himself. Agamemnon took possession of her and had two children by her. When he returned home, Clytemnestra not only killed Agamemnon, but also Cassandra. Once again her prophecy of evil had not been believed.

The name is commonly used for someone who is always predicting woe, though the Cassandra story is about someone delivering warnings which are not believed.