River Lethe

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 re-birth bathe in it to completely forget who their former selves are, allowing them to be reincarnated into another life.

The Sword of Hades
Percy, Thalia, and Nico 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 effected 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 it up 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 had time for one last thought: dry.

The Last Olympian
Nico and Percy see a vision of when Hades was talking to Maria di Angelo, in which Zeus destroys 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.

The Lost Hero
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've made him forget even more of his identity.

The Mark of Athena
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.

Trivia

 * 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 with.
 * The branch of a poplar tree with Lethe water dripping from it is the symbol of Hypnos.