Lethe is one of the five rivers of the Greek Underworld. It is known as the river of forgetfulness due to the fact that those who drink it will forget everything about their lives. Lethe is also the name of the spirit of forgetfulness, whom which the river was named.
The River[]
The other four rivers in the Underworld are Acheron (the river of sorrow), Cocytus (the river of wailing), Phlegethon (the river of fire), and Styx (the border river). Lethe borders Elysium and runs all the way to Hades palace, so that the spirits who choose to go onto another life after they die can drink from it and forget their previous life. Lethe is also said to flow through the cave of Hypnos, the god of sleep.
Those who wished to reach the Isle of the Blessed would reincarnate four times, and if they had lived all four lives well then they would go to the Isle. Once a person had lived those four lives and were deemed to be worthy of the Isle of the Blessed would be permitted to drink from the Mnemosyne (the pool of memory named for the Titan of memory). They would then remember all of their previous lives and would go on to live eternity in bliss.