The River Lethe is the river of forgetfulness and is one of several rivers in the Underworld. A mere drop from the river can make someone begin to forget their whole identity. Spirits who wish to be reborn bathe in it to completely forget who their former selves are, allowing them to be reincarnated into another life.
Percy Jackson, Thalia Grace, and Nico di Angelo cross it and the Titan Iapetus is thrown in it after Percy tosses himself and Iapetus in while they are fighting. Iapetus forgets all about who he is (Percy isn't affected because of his innate ability to stay dry underwater), and later Percy tells him he is on the same side Percy is on and that his name is Bob.
Usually it's easy for Percy to stay dry, but he states that this was different. As he held up the river for his friends, he could feel the river resisting him, wanting to erase their memories. In the end, everyone got out but Percy, who was getting weak from holding the river up. The river fell down on him, but he did have time for one last thought: dry.
Nico and Percy see a vision of Hades talking to Maria di Angelo, in a hotel. Here, they also see Zeus destroy Maria along with the hotel. Hades orders Alecto to dump Nico and his sister, Bianca, in the Lethe. He also tells her to hide them in the Lotus Hotel and Casino.
When Annabeth takes Jason to the Hypnos Cabin, Jason sees milky white liquid dropping from a tree branch in tin bowls, Jason was tempted to taste it, but luckily he restrained himself. It was then revealed it was Hypnos' symbol and that doing so would have made him forget even more of his identity.
The two twin giants, Otis and Ephialtes, briefly talk about the River Lethe while they argue. Ephialtes, in an attempt to gain media attention, had Otis suspended in ice over the River Lethe. Needless to say, the stunt did not work and they didn't gain any attention for it.
Iapetus, now known as Bob due to his memory being erased by the River Lethe, appears in Tartarus to help Percy and Annabeth escape. Percy learns that since becoming "Bob", he has been turned into a servant by Hades and feels guilt for the way he treated his friend. During the journey, Bob learns from the arai that Percy destroyed his memory and stops helping as a result. However, when Percy apologizes, he forgives him and helps again. Over the course of the journey, the regenerative effects of Tartarus on monsters along with encountering beings and places from his past causes the River Lethe's effects on Bob to start to reverse and his memory to start coming back. Eventually the Lethe's power is negated entirely and Bob regains his memory, but makes a choice to keep the personality he got as a result of the memory wipe rather than becoming Iapetus again.
- Nico states that swimming in the Lethe will turn your mind into that of a newborn baby. But the only character who has been seen dipped in the Lethe, Iapetus, was still able to speak when he arrived out. It is possible he was not submerged for long enough or because he was a Titan, his mind was strong enough to partially resist it.
- In Classical Greek, the word Lethe literally means "oblivion" or "forgetfulness."
- In The Demigod Files, the Lethe water was black, while in The Lost Hero, it was milky white.
- Lethe was also the name of the Greek spirit of forgetfulness and oblivion, with whom the river was often associated.
- The branch of a poplar tree with Lethe water dripping from it is the symbol of Hypnos.
- The effects of the Lethe appear to be temporary, as both Nico and Bob have shown remembering their past in The House of Hades. However, Bob's returned memory is stated to be the regenerative effects of Tartarus on monsters. Its doubtful this is why Nico remembers despite going there however as Bob states that Tartarus' effects are only beneficial for monsters.
|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|