Stygian

Heavenly ire / ne'er sent a pest more loathsome; ne'er were seen / worse plagues to issue from the Stygian mire.

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

Within all was black as the tomb. [...] Feeling before him upon the floor with the butt of his spear, Tarzan entered the Stygian gloom.

Stygians – natives of Styx