From Halopedia, the Halo wiki
Xellus keep was the head keep of the Sangheili state of Xellus on Sanghelios. Just inside of one of the keep's walls, there was a large plaza. Beyond the wall, there was rolling, steep hills. Tersa 'Gunok's clan had an ancient pact with the 'Xellus clan that allowed them to train at the keep.
In 851 BCE, Ussa 'Xellus warned the residents of the state to not trust the newly-formed Covenant or the San'Shyuum. Some of the keep's residents did not agree with Ussa and reported his heresy to Covenant authorities. In response, nine attack fighters were sent by the Covenant to fire upon the keep. The fighters dropped explosive charges on the keep's plaza, instantly killing numerous Sangheili. While the fighters attacked the keep, some Sangheili attempted to shoot them down with surface-to-air launchers. However, in the five bombing runs the nine fighters made, only Ussa succeeded in destroying one of the crafts. Ultimately, the keep was destroyed and hundreds of the keep's inhabitants were killed. However, the survivors were led by Ussa and they fled to the region of Nwari at Sanghelios' southern pole.
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