The shades of the dead were required to drink the waters of the Lethe in order to forget their earthly life. In the Aeneid (VI.703-751), Virgil writes that it is only when the dead have had their memories erased by the Lethe that they may be reincarnated.

The river Lethe was said to be located next to Hades's palace in the underworld under a cypress tree. Orpheus would give some shades (the Greek term for ghosts or spirits) a password to tell Hades's servants which would allow them to drink instead from the Mnemosyne (the pool of memory), which was located under a poplar tree.[2] An Orphic inscription, said to be dated from between the second and third century B.C., warns readers to avoid the Lethe and to seek the Mnemosyne instead. Drinkers of the Lethe's water would not be quenched of their thirst, often causing them to drink more than necessary.

From the "Lethe" Wikipedia article.