Go to file > Preferences and check Allow Negative values in Object editor.
Create an ability based off the item ability devotion aura. Turn item ability off, make it only target self, remove buff, change the art if you want, and set the icon to none.
Now create a spell that does nothing, preferably an aura and give it a buff.
Events: A unit is attacked
Conditions: Attacked unit has a specific buff equal to true (Boolean)
Actions: Add ability (the negative armor bonus) to attacking unit.
Now this will only reduce their armor to 0, you can't go into the negatives with it xD.