Via these triggers, but they only fire on the leader:

Code:
Trigger papalmission1
    WhenToTest LeaderMissionSuccess

    Condition MissionID return_papal_settlement

    Affects AdoredByPope  1  Chance  100
So they are not of much use to me, unless there is a fancy way to transfer a trait from a factionleader to the actual killer behind the scenes. There is the FactionLeaderTrait condition, but that still does not allow me to specify the killer.