Incredibly weird (I think) question.
In an upcoming campaign, one of my players will be playing an evil inquisitor who fanatically follows a god that orders him to secretly kill specific targets. To give this character a reason for joining the party, his primary target will be an NPC barbarian who is with the party and vital to the main quest.
Now, I will be (as the god) instructing the character up front that he should follow the barbarian for a while and kill him only when instructed. To be fair to the player though, I will, at some stage, give me the go ahead to try and kill him.
In a proper battle, the barbarian would slaughter him. The player's character is a talker, not a fighter. However, he is planning to do most of his killing at night when people are asleep and using a coup de grace with a x4 crit weapon.
Is there a barbarian power or archetype I can use to make this much more challenging (but not impossible) for the player? I'm happy for him to kill the NPC if he is truly able.