nightmarish

Concise Oxford English Dictionary © 2008 Oxford University Press:
nightmare/ˈnʌɪtmeə(r)/

▶noun

• 1 a frightening or unpleasant dream.

• 2 a very unpleasant experience or prospect.

– derivatives
nightmarish adjective,
nightmarishly adverb.

– origin ME (denoting a female evil spirit thought to lie upon and suffocate sleepers): from night + OE mære ‘incubus’.