Setup: For the NPC to provide assistance, the PCs need to convince him or her of their trustworthiness and that their cause helps the NPC in some way. The NPC is in a superior position (more troops/power/control/etc) to the PCs.
Level: Equal to the level of the party, depending on the power/influence of the NPC. For example, up to 15 for a regional baron, 20 for an emporer, or 30 for an extraplaner ruler.
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Intimidate (extremely hard): The NPC refuses to be intimidated by the PCs. Baring extraordinary circumstances, each use of this skill fails.