Stygian comes to us (by way of Latin stygius and Greek stygios) from Styx, the name of the principal river in Hades, the underworld of the dead in Greek mythology. This is the river over which Charon the boatman was said to ferry the spirits of the dead; the Greeks and Romans would place a coin in the mouth or hand of the deceased to serve as fare. It is also the river by which the gods swore their most binding oaths, according to the epics of Homer. English speakers have been using stygian to mean "of or relating to the river Styx" since the early 16th century. From there the meaning broadened to describe things that are as dark, dreary, and menacing as one might imagine Hades and the river Styx to be.
Recent Examples on the WebStreaks should start cutting through the stygian sky starting at midnight. Joe Mario Pedersen, orlandosentinel.com, 11 Aug. 2021 When his tiny friend Trinket devises a scheme for their escape from the orphanage, Arthur embarks on a quest through a wild wood and into the stygian corridors of a subterranean city to solve the mystery of his origins. Meghan Cox Gurdon, WSJ, 29 Sep. 2017 After a five-minute ride through the stygian darkness, the barge eases up to a low dock. Wayne Curtis, WIRED, 30 May 2017 There, Levi conjured a Stygian exoplanet of glissandos and microtones and processed percussive sounds, evoking the truly alien better than the most lavish special effects ever could have done. Adam Davidson, The New Yorker, 23 Feb. 2017 Far from being cruel and Stygian, the bog had a textural beauty, its atmosphere, like its fickle microclimate, changing with the wind. Henry Wismayer, New York Times, 16 Nov. 2016
Word History
Etymology
Latin stygius, from Greek stygios, from Styg-, Styx Styx