What was percy playing when he realized who his father was ?

a game where two teams would compete against each other to capture the opponents flag.

Explanation:

Percy Jackson was playing a game where two teams would compete against each other to capture the opponents flag. In this game, he badly gets injured when Clarisse and the four Ares warriors pull out a trick on him.

However, when Percy gets into the water, his wounds heal up immediately and a trident appears just above his head. At this sight, Annabeth and Chiron informs him that he was the son of Posiedon, one of the Big Three.