A Delusional Prophet

[AureusFulgens]

So I have a character in the mini-campaign I'm running this summer who is, like most of the members of his family, insane. His particular insanity is a complex set of delusions that make him believe the god Eispat (the god of kings) has ordained him a prophet and ordered him to assassinate his nephew, the king, so that his daughter can assume the throne.

He completely believes this delusion, so if he "comes out" to the players, any Insight checks would reveal that he's telling the truth (as far as he knows it). But should they be able to tell that he's delusional? My conception of the character is not a raving madman; he's a more calm sort of madman, so it isn't as though he's screaming "I AM A PROPHET, BOW TO ME!"

[holywhippet]

Let them draw their own conclusions. Have him tell them the truth as they believe it. They can conclude he is crazy based on what he is saying.

You could allow a religion check to note discreptancies between what he says and the actual doctrine of Eispat. You could also allow things like augury to indicate that he does not in fact have Eispat backing him up.

[kraitmarais]

When they make the insight checks (and presumably succeed), you just tell them that “your impression is that he believes what he’s saying.” An insight check alone doesn’t give them any impression of the actual truth or falsity of the underlying matter.

Intelligence (History) or other checks might reveal problems with his story; the players may also encounter other situations that suggest that the truth isn’t what he proclaims.

[Unoriginal]

They could meet a Cleric of this god, who by definition is a Chosen One and prophet of their deity, and realize that the delisional madman doesn't have any power associated with the position through talking with the Cleric.

[Honest Tiefling]

I think Holywhippet has good ideas. Insight checks are nice and all, but I doubt most people interested in story (and why would this villain be in a non-story campaign?) would really be upset if they couldn't solve all RP encounters with one dice roll. Having insight AND religion checks be important seems like a much better route. Having conflicting (or at least, conflicting at first glance) information and having to resolve it can set up a good mini-mystery.

I suggest also having this NPC be initially helpful to the party. This will make sure they want to interact with him, so you can reveal the truth slowly. Perhaps this individual knows that most disregard his holy mission, so he seems perfectly nice and calm at first, but as his plans are unraveled, he becomes more and more desperate, much like sane people. For him, he'd accidentally reveal his delusions and be very hard to sway from his version of the truth. Building on Unoriginal's idea, having a 'rival' and true cleric of Eispat be incredibly rude to the party might set up interesting interactions later, depending how the party handles it as well.