Delphi told Acrisius that the future son of his daughter will grow up and kill him. To avoid such a happening, Acrisius kept Danaen locked up in a chamber of bronze so no one could come near her.
He came up with a plan and persuaded young Perseus to fetch him the head of Medusa the Grogon,thus hoping to be rid of him for good as anyone who looked at her would turn to stone.
Once upon a time Medusa was a beautiful maiden, but she had an affair with Poseidon in one of Athena's temple. Angered by this disrespectful behavior, Athena changed the pregnant Medusa from the beautiful maiden into a hideous beast, but left her mortal.
Form the nymphs of the north, Perseus found out the secret of how to kill Medusa. They also gave him three special gifts: winged sandals that allowed him to fly, a wallet that would expand to hold anything of any size, and cap that would make its wearer invisible.
Out of revenge, Poseidon sent a sea monster to devour everyone in the city unless the king would sacrifice his daughter.
Many years later, Persus was participating in a discus-throwing contest. He accidentally flung the discus into the crowd an killed a spectator, who is, unbeknownst to him, Arcisuis, his grandfather. The original prophecy had been fulfilled; no one can escape fate.