River Lethe

The river Lethe is one of the several rivers of the Underworld. One drop of it, and you begin to forget your identity. Spirits who wish to start over bathe in it and completely forget who they are.

The Demigod Files
Percy Jackson, Thalia, and Nico must cross it in "Percy Jackson and the Sword of Hades". The Titan, Iapetus, is dunked in it and Percy renames him Bob.

The Last Olympian
Nico and Percy see a vision of when Hades was talking to Maria di Angelo. Zeus kills her and Hades orders Alecto the Fury to dump Nico and his sister Bianca in the Lethe, then put them in the Lotus Hotel and Casino to hide them from Zeus, because they are Maria's children with Hades. After the prophecy about a child of the Big Three was made, Zeus, Poseidon, and Hades had sworn not to have any kids.