It might be worth rereading the reference material for Secrets first.
OK. So: how to implement this in Reign? I’m thinking aloud, because pondering it in solitude for a week hasn’t gotten me much progress. Rough edges ahead.
Reign skills are on a 1 to 5 scale. A character’s knowledge of a Secret should cap the points they can have in that skill, as per the base model. Or, hm. Skill plus stat generally caps out at around 10, so perhaps Secret knowledge caps the dice pool? That provides finer-grained distinctions.
The poles: if one person knows it, the dice pool caps at 10. If a lot of people know it, the dice pool caps at 1. Filling in between them:
1: 10 dice
2: 8 dice
3: 6 dice
4-5: 5 dice
6-7: 4 dice
8-10: 3 dice
11-20: 2 dice
21+: 1 die
It’s still worth having more dice in your pool to overcome die penalties.
OK. Secrets provide control, vision, vulnerability, and blindness.
Blindness is fairly easy; it’s an opposed roll between Sight + Sense and your Secret plus your Knowledge skill. You want to win in Width to see the problem before it becomes problematic, in some situations. At other times, only Height matters. Possible variation: use the maximum dice pool instead of Secret + Knowledge. While I’m going to try to avoid perception rolls in general, it’s appropriate for this magical element.
I don’t know how to do the vulnerability yet. Some conversion factor for penalty dice, perhaps?
Control is a Secret + Knowledge roll. Consider using the magical rules from Supplement 3 to define the scope of control based on the number of people who know the secret.
Vision is a Secret + Sense roll, probably usually opposed.