Inclined to require purchasing Soulgaze.
Then have a high concept related to redemption of evil or vengeance or somethign else applicable.
I was thinking having it act as weapon 2 attack based on conviction (modified by intimidation?) defended with conviction. With a potential upgrade adding 2 more weapon value (only available to purchase once of course).
I'd allow the character to choose whether or not to penance stare while soulgazing, since this is a seperate power.
This would be a one way damage power (the defender does not attack back) requiring the same situations to initiate as Soulgaze. I would allow the user of said power to tag evil aspects of the victim for +2 to the roll. Like any other attack the attacker could make declarations (I just saw this person steal/murder etc.), evoke their own aspects, or tag/evoke consequences of the target.
To use those evil asepcts the character would have to learn them by other means : empathy, Soulgaze etc.
Much like Soulgaze, penance Stare should only be usable once per individual until they've hurt more people sinned substantially etc. (Much like Soulgaze only works once until a person changes at their core)
Penance Stare only works on beings capable of emotion or capable of being Soulgazed ( good idea?)
target recieves a temporary aspect scared straight which can be removed like any other aspect placed on them.
perhaps I;'d allow the custom stunt (you don't want to see my soul) add to this but it would get pretty mean as a weeapon 6 attack....
Cost: -3? (In addition to the -1 for Soulgaze) Optionally -1 for a weapon 2 bonus?
Very preliminary and in need of much revising.