So, lessee. Contributing factors for a catch:
Protects against everything except certain effects: +0
Catch can only be accessed by a limited set of people (assuming that, much like holy stuff, you need someone with actual True Belief in Evil to power such things - if this assumption is false, that's another plus one): +1
Researchability: +0 or +1, depending. I.e., if it's possible for someone to sit down and say "this kind of demonic scion is particularly vulnerable to unholy stuff", then you've got a +1 here. If someone would have to know you in particular and that you'd switched sides to figure that catch out, then this would be a +0.
So, total is somewhere in the range of +1 to +3, depending on the exact details. I'd expect it to end up at +1, but you haven't given enough information for me to be sure - see above conditional statements.