Cassandra was a daughter of Priam, king of Troy. She agreed to sleep with the sungod Apollo if he granted her wish to always prophesy accurately. Once the wish was granted she changed her mind about her side of the bargain. He couldn't take back the gift, so added to it that she should never been believed.
Among other things she prophesied that Troy would be destroyed and set on fire and the inhabitants slaughtered or taken as slaves. When she saw the wooden horse which the Greeks had left when they pretended to depart, she begged the Trojans not to accept it, but they jeered at her and dragged it inside the gates and held a feast to celebrate the departure of their beseigers.
In the night, while the Trojans were all drunk and asleep, ten Greeks crept out of the hollow horse and unbolted the gate to let in the rest of their army, who had sailed just over the horizon, out of sight.
She was right, but not believed.