Acheron n. : A river in the Nether World or infernal regions; also, the infernal regions themselves. By some of the English poets it was supposed to be a flaming lake or gulf.

Example:
What is the 1st book in the dark-hunter series that involves Acheron and his deal?

Taxonomy
Acheron, Acheron river, across the Acheron, free encyclopedia, ancient Greek mythology, Virgil, Acherusian, Lake Avernus, Greek mythology, Greek river, northwest Greece, mythology, five rivers, river of Hades, search, article concerns, the village, Glyki.

Defination from the web:
Acheron was known as the river of pain, and was one of the five rivers of the Greek underworld. ... He claimed that Acheron flowed in the opposite direction from Oceanus beneath the earth under desert places. ... Retrieved from "http://en.wikipedia.org/wiki/Acheron" ...