This unnamed Prophet was a San Shyuum of the Covenant Empire responsible for supply routes in an unknown sector of the Milky Way galaxy.
A world under Covenant control, Heian, a covert Human combat team, comprised of three Orbital Drop Shock Troopers (ODSTs) led by SPARTAN II Cal-141, managed to slip past the Covenant Army patrols and take up an overwatch position above a landing zone in a staging area.
Just before the Cal was able to shoot and assassinate the Prophet, after his arrival in a Type-52 Phantom, a Brute Chieftain snuck up on the team and injured her. During which, Cal ordered O'Brien to take the shot. After sighting in on the calm and unsuspecting Prophet, O'Brien managed to kill the alien from a far distance of nearly a mile and a half.
The Prophet's assassination and death allowed for the United Nations Space Command to regroup and exploit the new weakness in the Covenant supply chain.