How did Percy’s prophecy come true in Chapter 22?

Neither god can believe Ares would do such a thing, but Percy tells him he did not act alone. He explains about his dreams and about the powerful creature in the pit that felt older than even the gods. Zeus grudgingly thanks Percy for his work and disappears, leaving Percy with his father. Poseidon explains that in the pit is Kronos, king of the Titans and Poseidon and Zeus's father, and tells the story of how Zeus cut Kronos into a thousand pieces and banished him to the pit of Tartarus. Percy insists that Kronos intends to rise from the pit and come back to power. Poseidon then tells Percy he must go, but reveals that his mother has been returned, since he returned the helm to Hades. As Percy leaves, Poseidon tells him that he will soon have to make an important choice, and that he is proud of him.