The adjective “Stygian” is always capitalized because it comes from the Styx River, which is one of the five rivers of the underworld in Greek mythology. The Styx served as the barrier between the world of the living and the world of the dead, and when someone died, they had to cross the river, ferried across by the ferryman Charon. The stories say that for anyone else, the river was poisonous — there is a legend that Alexander the Great was poisoned by the Styx. The adjective “Stygian” came into use in the 16th century, originally used for references to the Styx River specifically, but it expanded to describe very dark, gloomy, or black things or situations.