Nightmare

noun

1. A demon or monster, thought to plague people while they slept and cause a feeling of suffocation and terror during sleep.
2. Sleep paralysis.
3. A very bad or frightening dream.

   "I had a nightmare that I tried to run but could neither move nor breathe."

4. Any bad, miserable, difficult or terrifying situation or experience that arouses anxiety, terror, agony or great displeasure.

   "Cleaning up after identity theft can be a nightmare of phone calls and letters."