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)

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