Stygian

Styg"i*an (?), a. [L. Stygius, fr. Styx, Stygis, Gr. , , the Styx.]

Of or pertaining to the river Styx; hence, hellish; infernal. See Styx.

At that so sudden blaze, the Stygian throng Bent their aspect. Milton.

© Webster 1913.