The Lethe patch is a patch for NetHack 3.3.1 by Michael Clarke.
It adds several new monsters (including some from the Cthulhu Mythos) and amnesia-causing water in potions and moats.
For the SLASH'EM version see SLethe.
Lethe means forgetfulness in Greek(Λήθη). The lethe river was one of the five rivers of Hades(the underground kingdom of the dead in ancient Greek mythology). Who ever drank from its water, was struck with complete forgetfulness. Hence the lethe river with amnesia causing lethe water flowing through the lethe gorge and in to Gehennom.