Harpies don't have a set territory, but they generally make their homes in the Silver Mountains. This is the larger of the two mountain ranges in the Mirror Realm and the one which divides the Light and Dark Elves from everyone else. There are three paths through this range, but only one is wide enough for, say, two full grown Centaurs to walk side by side without the danger of on falling off the side. This is also the only one useable during the colder months as the other two become covered in ice and snow.
