you would need fire/astral gems like the ark, just need h magic to forge it.
Maybe instead of "destroy evil," the effect would be to "save soul" or "release soul" allowing the undead or demon to RIP.
A weapon to kill a tartarian instantly would be a good thing-though i do not think they should be common.
Another good counter to sc's would be to add "arm loss" to a constr 6 2h weapon like the gate cleaver, and a "limp" to a constr 6 2h weapon like a Scythe that would seem to be used as a sweep at the legs.
A legless or armless sc would be of limited value!