River of Lethe

Picture taken from this site here

In the underworld, there is a river of oblivion called Lethe. The shades of people who had passed on had to drink from it and would forget about their lives in the living world.

I got really excited when it was mentioned in J.K Rowling's Harry Potter (it's required in a forgetfulness potion) and in L.J Smith's Enchantress/ Spellbinder (Thea and Eric have to drink it to forget about the Night World).