From Halopedia, the Halo wiki
In the early years of the war, the Covenant attacked viciously and without mercy. Kholo was largely ignored in this early stage, until an unnamed Shipmaster found a freighter that hailed from the world, interrogated the pilot, and found the coordinates. After receiving holy approval from the High Council, he ordered the Fleet of Righteous Vigilance be sent to the human freighter's home system.
Kholo's defenses were considered weak by the Covenant, and the scout ships were able to inflict considerable damage before the main fleet even arrived. The Shipmaster was accompanied by the Prophet of Conviction, a San'Shyuum sent with him to complete a particular holy task. After removing the final layers of human resistance, the Shipmaster was given the command to glass the world, albeit in a special manner. Manually controlling the cleansing beam, he fired upon the planet's surface, etching a Covenant symbol into its crust, the symbol meaning the word, "Faith". The Shipmaster was subsequently deemed worthy of being bound to the Prophet, and the world was partially glassed by the rest of the fleet.
Later, six years after the Human-Covenant War, the Shipmaster returned to the world, trying to find purpose in his life and some atonement for his sins in glassing the world. There, among the bones of his victims, he found Kig-Yar and humans fighting over relics that were obviously Forerunner. Upon finding a wounded human survivor, the Shipmaster gave command to bring down a more permanent force, to learn more of the relics.
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