The phrase "nightmarish quality" refers to something that is very frightening, disturbing, or unsettling, like a nightmare or bad dream. Full definition
This unnerving compendium has made the transition from a successful international stage show (whose fanbase includes the director John Landis) to the big screen and draws inspiration from a rich history of horror, recalling the chilling and nightmarish quality of films including the Amicus anthologies and 1945 portmanteau, Dead of Night. (lwlies.com)
A number of artists replaced or altered its contents: Fred Tomaselli's ongoing series of reworked illustrations lend a vivid nightmarish quality to news stories about Hurricane Katrina and the Bernard Ebbers WorldCom fraud scandal. (artinprint.org)
But encroaching on the playful actors is a halo of darkness, and the masked figures exude a menacing, nightmarish quality as in the photographs of Ralph Eugene Meatyard.» (lightwork.org)