"Boys, if I were you, I'd back off. We've been hired to kill anyone who steals from this shipment. Now, if you cooperate with me, you have my word of honor as a Knight of the Platinum Dragon that I will ensure you are fed. We do not want to kill you, but if you try to steal from us, we will have no choice. If I have to, I will personally buy each of you a meal if you will desist and help us reach the city safely."
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charisma check. If it's supposed to be a specific skill check, lemme know which one and I'll edit to indicate the proper modifier.
(d20+3)[19] modifier should be +7 for diplomacy, as discussed. Thus, my total is 23.