Prophecies play a major part in Greek mythology. They are the indirect cause of many of the heroes' journeys and play a part in every myth. The oracle always prophesizes how the subject will die, and then they go to so much trouble trying to prevent the prophecy from being fulfilled, that they bring it about themselves. Jason, Perseus, and Zeus were all destined to kill a certain person, and they were all almost stopped, but with help from the gods they carried out the prophecy.
Jason's father was the king, but his throne was usurped by his uncle Pelias. Jason was determined to take back his kingdom. His uncle had been warned that he would be overthrown by a one-sandaled stranger, and Jason lost a sandal when he was carrying Hera across a river. When he arrived, King Pelias did not want to just give up his power, and when he saw Jason's single sandal, he tried to put him in the position of being killed by sending him to bring back the Golden Fleece.
He completed his task, and when he returned he tricked Pelias' daughters into killing him, and Jason fulfilled the prophecy that he would bring about the demise of king Pelias.
King Acrisius had a daughter named Danea. He learned from the oracle of Apollo that he was going to be killed by Danea's son. To prevent this, he locked her in a dungeon where no one would find her, but Zeus came to her, and she had a son named Perseus. Perseus led an interesting life, and compulsively promised Polydectys Medusa's head, he completed the task, and used the head to turn Polydectys into stone. On his way home, he entered an archery contest and his arrow...