In Exalted, Lethe is a state of peace entered by the Dead when they decide to abandon the Underworld and reincarnate. When a ghost decides to enter Lethe, they fade away over the course of 3 days, during which time no force in creation can interfere. At the end of the third day, the ghost dissolves, leaving one final message to any being in creation.
Because Lethe is available to any free ghost (those ghosts who enter into and choose to remain in bondage cannot enter Lethe unless their master frees them), ghostly society has generally been fiercely independent. It is always possible to opt out of death and try again on the wheel. With the arrival of the Deathlords, this has changed - the Deathlords have found that Soulsteel chains will instead cause a ghost to enter Oblivion rather than Lethe, which has made mass enslavement a reality in the underworld.
Before the Underworld existed, souls immediately went through Lethe upon death. Unfortunately, when the Exalted killed a Primordial for the first time, the almighty being's souls were too powerful to be recycled, turning it into the first Neverborn, and creating the Underworld.
