From Halopedia, the Halo wiki
In 851 BCE, Ussa 'Xellus warned the state of Xellus' residents to not trust the San'Shyuum or surrender to the newly formed Covenant. Some of the state's residents fled and reported Ussa's "heresy" to the Covenant. The next cycle, N'oraq was on the walls of Xellus keep carrying out a training exercise with Tersa 'Gunok and another friend. During the exercise, Tersa spotted nine Covenant fighters sent to attack the state for Ussa's crimes. Tersa warned N'oraq of the imminent attack and told him to sound the keep's alarm. However, N'oraq did not see the fighters and thought it was part of the exercise. As Tersa sounded the alarm, the fighters began firing at the keep. Explosive charges dropped on the walls and surrounding areas tore N'oraq in half, killing the Sangheili youngling. The death of N'oraq fueled Tersa's hatred for the Covenant and all of its members.
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