I have worked with one of my players on a PC martyrdom scene that involves the group needing to steal a staff of power from a wizard and then one PC breaking the staff to save the rest of the group. (The player will soon be leaving the campaign and wants his character to go out with a big bang!)
Here's the issue: The PCs are all third and fourth level, which means according to the NPC equipment chart, nobody they fight at this level should have access to such an item. I'm not worried about the item throwing off the balance of the campaign because it will be destroyed in the same session they receive it. What I want to know is: How much should the challenge rating of an NPC should be adjusted for having equipment that is far above his level? I'm thinking this NPC will probably be somewhere in the range of 5th to 7th level, but I'm not sure how much difference the staff will make. Obviously it makes him stronger... but by how much in terms of CR?
I can always just eyeball it, but I was wondering if there were any official guidelines anywhere? or at least someone who has fiddled with the idea already?
Thanks ahead of time!