From Longman Dictionary of Contemporary EnglishStygianSty‧gi‧an /ˈstɪdʒiən/ adjective [usually before noun] literary DARKunpleasantly dark, and making you feel nervous or afraid the Stygian gloom
Examples from the Corpus
Stygian• Stygian caverns• No green flash, nothing, and the sky above fading from blue through green to the beginning of Stygian darkness.• She lowered her head and entered the Stygian darkness.• We navigated the Stygian gloom of the corridor arm in arm like a quaint, old-fashioned couple.Origin Stygian (1500-1600) Latin stygius, from Greek, from Styx river in ancient Greek stories which people cross over when they die 