What is the proof in the first part of the quest?

Why don't you describe the quest to us so we know what you're talking about.

Assuming you mean the quest where you must prove who's behind the liquor problem in the fort, ask around about the guy, paying particualr attention to where both his room and his office are. Then go search them.
And hopefully that was vague enough to avoid being a spoiler.

Go to the top of the armory at the fort in there is a room there unlock the desk (careful not to get caught) take all the liqour, then talk to carius. he will say if you have proof confront Antonius Nuncius about it.



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