The Korath desert lies far to the south and east of the Northern Kingdoms, beyond Tir Tochair, between the Nilfgaardian Empire and Zerrikania and the Haakland. Ciri wandered through this desert after her escape from the Thanedd Island during the coup through the portal in Tor Lara.
Because the portal was unstable, the girl was simply ejected in mid-air over the desert. When she finally regained consciousness, Ciri had no idea where she was, nor how to get out of the desert. She had no food nor water with her and her only defenses were one small dagger and her wits. She did meet a young unicorn and together they were better able to proceed than alone.