Cassandra

Cassandra, daughter of the Trojan kings Hecuba and Priam, was a priestess of Apollo, with whom she had made a pact: in exchange for a carnal encounter, she would receive the gift of prophecy.

Cassandra predicts the fall of Troy. Bernard Picart (1731), Rijksmuseum.
Cassandra predicts the fall of Troy. Bernard Picart (1731), Rijksmuseum.
Source: Wikimedia Commons. CC0.
Cassandra. Frederick Sandys, Ulster Museum.
Cassandra. Frederick Sandys, Ulster Museum.
Source: Wikimedia Commons. Public domain.

When Cassandra obtained the faculty of divination, she broke her promise to yield to the sexual favors requested by Apollo, who, feeling betrayed, decided to punish her: without taking away her gift, he transformed it into a curse. Cassandra could see the future, but no one would ever believe her predictions. Thus, when she announced the fall of Troy, no one took her prophecy seriously.