An impasse
No symptoms of the plague are obvious.
"No. I not leave my young ones. I do not risk being sick and falling to demon's magic. You back to far wall. We drop our weapons and treasure. Attack us and you get sick as well. You not die if you let us go, you die if you don't. Understand? You deal with demon in mines."
M'dok makes several motions with his hands and his breathren simultaniously drop their weapons, armor, and assorted other equipment. He sends a kobold farther back on the ledge out of sight; it returns a minute later with two young kobolds. He then orders the kobold on the cavern floor to return to the ledge, carefully skirting around the edge of the cavern so as to avoid immediate contact with the party.
M'dok points to the tunnel entrance to the northeast where the 'demon' supposedly resides. "Stand there. We exit there." He points to the sloping tunnel from which you came. "Agreed? We both benefit, no?"