One of the five rivers of the underworld of Hades, also known as the river of unmindfulness. Everyone who drank from it experienced complete forgetfulness. The word literally means 'oblivion', and is related to the Greek for 'truth' (aletheia) which means 'unforgetfulness'. So there is some kind of connection between Lethe and concealing the truth. At the same time, if the shades in Hades didn't drink from Lethe (and thereby have their memories erased), they would never have the chance to be reincarnated.

In Greek and Roman mythology, Tantalus, a lesser god, were, was guilty of a crime against the the gods themselves, and so was sentenced to eternal life in Tartarus, the one region of Hades where those guilty of the worst crimes are punished (Tartarus, not Hades,ican be equated to the Christian hell). Tartarus' punishment was eternal starvation - being bound in a location where overhanging fruit trees tempted him, but moved out of reach whenever he reached for fruit. From Tantalus, we get the word "tantalize".

Milton's concept of Hades apparently does not include the happy Persephone, who ruled 1/3 of the year with her husband Hades (Pluto). The few Greek heroes who were able to descend into Hades found Persephone quite satisfied with her life there, her husband and her role as queen. She may even have chosen to eat pomegranates in order to stay there part of the year and not return to her mother!

Milton takes the ancient Greek geographical concept of Hades and its rivers - when Hades is not at all like hell -and uses it as basis for the Christian hell (maybe in part because he and many Christian leaders placed pagans and others of virtue who lived before Christ in the Limbo between Heaven and Hell).

Wikipedia has an article on the Greek underworld Hades at https://en.wikipedia.org/wiki/Greek_underworld Wikipedia also has a page on Christian views of Hades. https://en.wikipedia.org/wiki/Christian_views_on_Hades The rivers that Milton mentions exist in the Greek view of hell but with some slight differences. For example, Styx is not "abhorred Styx the flood of deadly hate". Gods swore unbreakable oaths on the River Styx, whose waters were believed to make one invulnerable to death.