The
adjectival form of
nightmare, so,
like a nightmare.
Ghoulish,
ghastly,
horrifying, possibly
surreal,
disorienting or maybe characterized by unwelcome
clarity; depends on what nightmares are to you. Generally used of a real-world thing or situation, as if a dream is nightmarish, you might as well just say it was a nightmare! Of course, this does not preclude you from using
nightmare as a
metaphor instead of using the adjective: do what you want.
The atmosphere in the club was nightmarish. (adj)
That hairdo is a nightmare! (metaphor)