Stygian Definition

Stygian
adj

1. Belonging, referring or relating to the river Styx, which, in Greek mythology, was the river in Hades across which the souls of the dead were ferried by Charon.
2a. literary
Dark and gloomy;
2b. literary
Hellish;
2c. literary
Said of a promise, oath, etc: strongly binding.

Etymology: 16c: from Latin Stygius, from Greek Stygios, from Styx Styx.