Random drop from sentries guarding entrance to Nilfgaardian Quarter in Chapter 3.
On Roche's Path, this sword can be found in the Nilfgaardian Camp immediately after saving Triss (should you decide to save Triss instead of Anaïs). After leaving the dungeon Triss was held in, run through the large door leading to the exit. In the next area, run forward and you should see a ladder on the other side of a fallen wall. Turn right and the Harpy will be in a chest among other items.