Sadly, the maddest and most self-assured of egomaniacal villains are exceedingly difficult to judge by this artifact. But then, perhaps nothing else can stop a certain nefarious antagonist except the black sword of the crown?
I can think of something to add - a note of how the willpower "test" is affected by circumstances: so may the life-giving judgment be more difficult as time passes from the death of the one to be restored. The more powerful the life-force of a target, the more difficult it is to snuff out with the black flame. Etc.