User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-9703010-20130919153421

Phlegethon:

In  Greek mythology, the river  Phlegethon  ( Φλεγέθων ,  English translation : "flaming") or  Pyriphlegethon  (Πυριφλεγέθων, English translation: "fire-flaming") was one of the five rivers in the infernal regions of  the underworld , along with the rivers  Styx ,  Lethe ,  Cocytus , and  Acheron. Plato  describes it as "a stream of fire, which coils round the earth and flows into the depths of  Tartarus ." It was parallel to the river  Styx. It is said that the goddess Styx was in love with Phlegethon, but she was consumed by his flames and sent to Hades. Eventually when Hades allowed her river to flow through, they reunited.

Cocytus:

Cocytus  or  Kokytos, meaning "the river of wailing" (from the Greek Κωκυτός, "lamentation"), is a river in  the underworld  in  Greek mythology. Cocytus flows into the river  Acheron, across which is the underworld, the mythological abode of the dead. There are five rivers encircling Hades. The River  Styx  is perhaps the most famous; the other rivers are  Phlegethon ,  Lethe, and  Acheron.