Cleon
Legend
Yes, something like that. Following the precedent of the barbazu's Infernal Wound, I guess we'd either put this in the weapon description or as a separate SA.
I prefer it as a Special Attack, it's clearer to keep them separate:
Shadow Wound (Su): Any creature the Shadow Walker injures with its scythe-chain must succeed at a DC X Fortitude save or take 1d4 Con damage. The shadow wound is a supernatural ability of the Shadow Walker, not of the weapon. The save DC is Charisma-based. [?]