What does the oracle tell King

Acrisios?

What? Because I have no idea what is the joke

That That he would be overthrown by a future son ...

Explanation:

An oracle told king Acrisios told his future and how his daughter will have a kid that will kill him. your grandson will kill you. King Acrisios imprisoned his daughter to prevent the birth of his grand son and his death. After his daughter had a baby he put them in a box and set them off to sea to drown or starve.