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. It adds a new branch, the Lethe Gorge, between Medusa and the Valley of the Dead and features a completely revamped Gehennom. For the SLASH'EM version see SLethe.