an extremely difficult or frightening situationThe journey to work was a nightmare.
nightmare of: The company now faces the nightmare of bankruptcy.
absolute nightmare: Heavy rain made playing conditions an absolute nightmare.
administrative/bureaucratic etc nightmare: The new tax has been a bureaucratic nightmare.
someone’s worst nightmare: The situation on the roads that day was every driver’s worst nightmare.
nightmare scenario (=the worst possible situation that you can imagine): If this computer system fails, this could turn into a nightmare scenario.