Say we have a situation in which a PC is caught between a couple Nightmare factions, siding with one against the other. In a conflict, he's assisted by one Nightmare against a crew of other Nightmares ... how's that work? I was sort of thinking it'd be best to reduce the Pain pool a bit, since the task is theoretically made easier, but it was also suggested to add to Discipline, just as it's done when a PC aids another PC.
Heh.
Your core question when it comes down to it should be, "How much does that hurt?" -- and set Pain based on that.
If the Nightmare "helping" is actually a way of saying "the ally Nightmare has more control over this situation than the PCs do" -- then I'd actually say it *increases* Pain. Even if the enemy-Nightmares get knocked down a peg, that's to the ally-Nightmare's advantage, not the PC's.
If the ally-Nightmare is actually trying to give the PCs more influence over things, then I'd say it decreases Pain.
Dig?
In this particular case, it's just a case of kicking some Nightmares' teeth in, but yeah - I read you.
For the sake of clarity - the PC is starting the fight and the 'friendly' (for the moment) Nightmare may or may not be backing him up. That actually remains to be seen.
I'd add madness. Why should a nightmares assistance reduce pain or add discipline? It just makes sense that the cost of their help would be increased madness.