Final answer:
In The Lightning Thief, Percy, Annabeth, and Grover meet Ares, the god who has turned, in the prophecy. This event occurs in Chapter 21 of the book.
Step-by-step explanation:
In the prophecy from the beginning of The Lightning Thief, Percy, Annabeth, and Grover meet the god who has turned out to be Ares, the god of war. Ares is shown as betraying them by stealing the Lightning Bolt and framing Percy for the theft. This event takes place in Chapter 21, titled "We Say Good-bye, Sort Of."
Learn more about Characters and Events in The Lightning Thief