River Acheron is the fifth river of the Underworld. It is called the River of Pain as it is the ultimate punishment for many souls of the damned, especially murderers. This is the point where the daemon, a divinity or supernatural being of a nature between gods and humans, Charon, ferries the souls of the dead to the Underworld.
The Acheron is described as being about twenty feet across, with a roaring current that has thousands of voices crying out, shrieking in agony and pleading for mercy. The Acheron also has a malevolent and mesmerizing effect on whomever comes too close to it, blaming them for egregious sins they never committed, and showing them their worst memories, trying to convince them to jump into the river.
After escaping Nyx and her children through the Mansion of Night, Percy Jackson and Annabeth Chase reach the banks of Acheron. They hear the river's malevolent mesmerizing voices, and Annabeth's mind is "flooded" with images of all of the monsters that she had ever killed. Later on, Annabeth sees Zoë Nightshade, Bianca di Angelo, Michael Yew, Silena Beauregard, and, most painful of all, Luke Castellan, along with his blood on her dagger. The Acheron claims that she could have prevented their deaths, and that Luke's blood is entirely on her hands, making Annabeth the actual murderer. She begins to fear that Luke's voice is among the those in the Acheron, punished for his crimes against Olympus. The river tries to force her to jump in and "share his punishment", and Annabeth nearly does, though Percy intervenes just in time. Just as Nyx's fearsome children are catching up to them, Percy grabs Annabeth and leaps over the Acheron, and manages to escape Night's territory. Afterwards, they both continue their journey into the Heart of Tartarus, the location of the Doors of Death.
|Underworld:||Chaos | DOA Recording Studios | Doors of Death | Door of Orpheus | Elysium | Erebos | Fields of Asphodel | Fields of Punishment | Hades' Palace | Isles of the Blest | Judgment Pavilion | Persephone's Garden | River Lethe | River Styx | River Acheron | River Phlegethon | River Cocytus|
|Tartarus:||The Mansion of Night | Hermes' Shrine | Chaos|