Prior to a visit to Heian, a world under Covenant control, a covert Human combat team, comprised of three Orbital Drop Shock Troopers led by SPARTAN-II Cal-141, managed to slip past Covenant Army patrols and take up an overwatch position above a landing zone in a staging area.
Just before the SPARTAN was able to shoot the Prophet after his arrival in a Type-52 Phantom, a Jiralhanae Chieftain snuck up on the team and injured the veteran sniper. During which, Cal ordered O'Brien to take the shot. After sighting in on the unsuspecting Prophet, O'Brien managed to kill the alien from a distance of nearly a mile and a half.
His death allowed for the United Nations Space Command to regroup and exploit the new weakness in the Covenant supply chain.