I'm not sure what sword you're looking for.
I don't remember which one was in the desert.
But I can tell you where they are.
Near where Rynn rescues the boy, there is a pile of crates. Climb them and jump to the ledge near the top of the pile. The first plinth is on that ledge.
Another one is close to the farm, inside a damaged house.
The final one is near Gothraul's throne, just to the right before you reach the stairs (it's very close to the door you exit as you come into the open).