The Duchy of Aotrom is referenced only in history books, old maps, and some bardic songs (but not very many). There are none alive which remember the Duchy, even among the long lived elves of Silva. From all accounts, the Aotrom was a thriving member of the Twelve Kingdoms until about 40 years ago; at least, no younger references to it have been found.
The duchy allegedly existed between Silva and the Twelve Kingdoms along Elfroad - the highway that connects the human Twelve Kingdoms to the elven lands of Silva. The lakes and other geographic features that should be there, according to the old maps, do not exist. In fact, the distance between the two kingdoms is far shorter than all of the older maps suggest - as if the duchy were simply folded away.