One day it is publically revealed how to do a ritual that summons a nonunique demon, devil, or Yugoloth(3.5 D&D/Pathfinder). The fiend can then be bargined with to do a favor in exchange for something from you(what that is would depend on the fiend). However they will only show if you truly wish to make a deal. One won't show just cause it was called and they can leave at any time.
How does humanity react?