I have just the right serum in mind. It will deal with the truth in precisely the way the truth should be dealt with.
For this concoction I will need several Shadow Panther hearts from the Swamp. I also require the enchanted fungus off of the Mire Lord who resides there. I am sure one as able as you, <name>, can persuade him to part with some.
The last item I'll need for the serum is the blood of a dreaming whelp, like the ones that live outside Itharius' cave in the swamp.
That Infiltrator in Stonard will surely.... open up... after he drinks my special serum. Haha!
The ingredients required are by no means easy to procure. Shadow Panthers are common in the Swamp but collecting enough hearts for our purpose can be daunting. And the Mire Lord can be such a fussy beast.