The sword Deithwen can be looted from one of the skeletons near a ruined wagon, a short distance past the entrance to the caves east of Loc Muinne. It is guarded by two arachas.
In Roche's path of Chapter III, a second copy of the sword appears in a cave by the forest path where Geralt can escort Brigida through during the quest Crown Witness.
This silver sword can be found as part of the quest Sunken Chest in the ruins on the small island between Byways, Frischlow, and Olena's Grove. To find it requires diving into the water, it is in the third chest from land as lot of sword, it has three rune slots.