Thanks Micah! I did not know the Oath Rod province-feeblemind effect was friendly fire only. How disappointing!
As far as using it in battle goes, it looks like the Feeblemind effect is not unresistable:
318 striking with weapon Oath Rod. att26 def23
hitloc Slave Prince strikes Ice Devil wl4 diff-2 -> 4
hitunit 318 252 dmg5 spec3 ba4
damage 15 on Ice Devil, spec0x3 ba4
hitloc Slave Prince strikes Ice Devil wl0 diff1 -> 2
hitunit 318 252 dmg33554432 spec131072 ba2
ustatbox unr252 sel0xbfad226c