Lethe

The daughter of Eris, Lethe was the goddess of oblivion and the river with the same name. When someone died and went to Hades, they had to drink from her water so hey would forget their previous existence on earth. If ever anybody was allowed back to life, again hey had to drink from the river so they would not remember the afterlife.

Lethe was the sister of Thanatos (death) and with Zeus she had the Graces/Charites. According to some myths she was the mother of Dionysus.