Perseus

what happens at the end of the story that causes Apollo's oracle to come true?

Perseus accidentally kills his grandfather.

Explanation:

When Perseus was born it was prophesied that he would kill Acrisius. It was after killing Medusa and going through several adventures guided, demanded and sometimes protected by some Gods he returns to his mother to her native Argos, where he accidentally struck her father, and kill him when throwing the discus, this event made the prophecy come true in an ironic turn of events.

Perseus accidentally killed his grandfather, Acrisius in a discus throwing contest and fulfills the Oracle prophecy.