Diamitriis’ fabled Island Fortress. Never confirmed to have existed, Orisetia was rumored to be the location where Diamitriis first bested the dragons in single combat, thus turning them to his side. They offered their nesting grounds as a secret base where Diamitriis and his closest allies could plan their campaings in peace.
The party has fairly good coordinates for this from the poem, but they do not make sense. If the coordinates are correct, then the islnad is either gone or incredibly well hidden.