(adjective) extremely dark, gloomy, or forbidding; refers to the River Styx of the underworld Hades in Greek mythology;
They come by infernal and Stygian IRT subway
stygian darkness
Everyone has an air of déjà vu about them, and they float like shadows over the waters of the Styx.
oddly gorgeous prose
she and Sam made their way down a Stygian stretch of Amsterdam Avenue
Bathed in this warm glow, a young woman named Ashli lay sleeping, impervious to the stygian immensity of her surroundings