I don't see it listed here, but when you make an action that doesn't have a relevant score to add to the d20 roll, what do you choose? Anything you want?
See here:
Abduct Peasants (violent action): You need peasants
(lvl 1-3 commoners) for your gruesome experiments or
perhaps as monster chow. Roll a Ruthless Check.
Success: You abduct 2d6 Peasants
Failure: You abduct no one.
Natural 1: Failure and -1 to Secrecy.
Natural 20: Success and +1 Ruthless.
As you can see that calls out that you must use a ruthless check.
But this:
Grave-Robbing (skilled action): Your organization
violates graves to acquire corpses and skeletons for you.
Success: 2d6 corpses or skeletons are acquired.
Failure: You are revealed. -1 to Secrecy
Natural 20: 4d6 are acquired instead.
Natural 1: As failure, but a number of minions
are caught and hanged. -1 to Survivability.
Doesn't say anything.