stygian

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.