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.
Deithwen returns as a silver sword in the third game and can be found as part of the Sunken Chest quest. This is located on the lone island south of Fyke Isle and north-northwest of Frischlow.
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