The River Cocytus, sometimes spelled River Kokytos, is a river found in the Underworld. It is also known as "The River of Lamentation".
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.
Cocytus was described as a river, flowing in a vast cavern deep in Tartarus, in which the entire island of Manhattan could fit. Red clouds hung around in the air like vaporized blood. The landscape was rocky black plains, punctuated by jagged mountains and fiery chasms. The river itself was a ribbon of glittering, black liquid with a smell of sulfur. It also appears to have a mind of its own, trying to attack Percy Jackson and Annabeth Chase. Cocytus' water is freezing cold, filled with millions of heartbroken voices, as if the river were made of distilled sadness. If dipped in, the person's thoughts will become sad and miserable, having no purpose to live.
Annabeth and Percy finally fall into the river after falling for a long period of time. Percy tries to create a whirlpool to get them but out, but falls under the spell of Misery. Annabeth tells Percy to start talking about New Rome and their life together there, while Annabeth drags Percy and her to shore. Eventually, they reach the shore and escape the freezing water.