Synonyms of the word 
NIGHTMARISH → ALARMING - BLOODCURDLING - HAIR-RAISING
nightmarish
- adj. Resembling a nightmare.
alarming
- v. present participle of alarm.
- adj. causing apprehension, fear or alarm; frightening.
bloodcurdling
- adj. Causing great horror or terror.
hair-raising
- adj. Causing fear; scary.
If you are interested in words, visit the following sites :
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- 1word.ws/nightmarish to play with words, suffixs, prefixs, etc.