Stygian
extremely dark, gloomy, or forbidding
The "dark and gloomy" sense of stygian is a figurative one, as the original meaning of the word (which may also be found in capitalized form) was decidedly literal ("of or relating to the river Styx"). This was the river presided over by the boatman Charon, who would ferry across the souls of the dead on their way to the underworld of Hades. The literal sense began to be used in the early 16th century, and by the beginning of the 17th had taken on its figurative sense.
Now mischief, murder, wrath of hell drawth nere
and dyre Phlegethon flood doth blood require
Achilles death shalbe reuenged here
With slaughter such as Stygian lakes desyre
Her daughters blood shall slake the sprites yre,
Whose sonne we slew. wherof doth yet remayne,
The wrath beneath, and hell shalbe theyr payne.
—Lucius Annaeus Seneca (trans. by Jasper Heywood), The Sixth Tragedy, 1559
