Udûn was a circular valley in Mordor between the Carach Angren and the Morannon, formed by a violent volcanic upheaval. The valley gradually rose in altitude from the level of the Anduin basin to the plateau of Gorgoroth. Udûn was a bleak place, without trees or beast. Caves in the steep walls and cliffs of the vale were home to small tribes of Orcs and Trolls.
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