From Halopedia, the Halo wiki
Nwari was a barren region on Sanghelios, near the planet's southern pole. The region was cold from the winds that originated in the south pole and a large, dormant volcano was located in southern Nwari. While the hills of the region were desolate, the caverns under the surface were warm due to the volcano heating the springs located throughout the caverns. Nwari was not well-known among the Sangheili.
After the state of Xellus was attacked by the Covenant due to the state's resistance of the Covenant and the San'Shyuum, Ussa 'Xellus and his clan fled to the volcano's crater in southern Nwari. The clan hid inside the region's caverns and had sealed off all but one entrance to prevent discovery. Ussa and his clan had collected several vessels and hid them in the volcano to use for their rebellion. Covenant spies and assassins were sent to the region to look for the clan. Fearing that Covenant forces were closing in on their location, Ussa and his clan eventually departed the region for the Refuge.
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