stygian

adjective3

1. •
   (literary, figurative) Infernal or hellish.
2. •
   Having a luminosity below 0%.
3. •
   (literary, figurative) Dark and gloomy.

   • The things which the Stygian darkness hid from my objective eye […] — 1913

adjective2

1. •
   Of, by or relating to the river Styx in Greek mythology.

   • But then, what though the mystic Three Around me ply their merry trade? — And Charon soon may carry me Across the gloomy Stygian glade? — 1897
2. •
   Of, by or relating to the Plutonian moon named after Styx.

noun1

1. •
   An inhabitant of Styx (a moon of Pluto).