One of the rivers of the Underworld introduced in Chapter 23. A thick fog hangs over the river most of the time, rendering visibility across to near zero. As a result, most travellers seeking the Underworld must wait for the ferryman, Charon, to guide them across the river to the Underworld.
It is noted that anyone who wishes to enter the Underworld must cross one of the rivers of the Underworld, regardless of the method taken to get there. Also, the crossing of these rivers signifies cutting one's ties to the Human World and crossing into the afterlife.